All of the above in the previous paragraphs allows us to formulate the basic rules for the integration of rational fractions.

1. If a rational fraction is improper, then it is represented as the sum of a polynomial and a proper rational fraction (see paragraph 2).

This reduces the integration of an improper rational fraction to the integration of a polynomial and a proper rational fraction.

2. Factor the denominator of the proper fraction.

3. A proper rational fraction is decomposed into the sum of simple fractions. This reduces the integration of a proper rational fraction to the integration of simple fractions.

Let's look at examples.

Example 1. Find .

Solution. Below the integral is an improper rational fraction. Selecting the whole part, we get

Hence,

Noting that , let us expand the proper rational fraction

to simple fractions:

(see formula (18)). That's why

Thus, we finally have

Example 2. Find

Solution. Below the integral is a proper rational fraction.

Expanding it into simple fractions (see formula (16)), we obtain

The problem of finding the indefinite integral of a fractionally rational function comes down to integrating simple fractions. Therefore, we recommend that you first familiarize yourself with the theory section of the decomposition of fractions into the simplest ones.

Example.

Find the indefinite integral.

Solution.

Since the degree of the numerator of the integrand is equal to the degree of the denominator, we first select the whole part by dividing the polynomial by the polynomial with a column:

That's why, .

The decomposition of the resulting proper rational fraction into simpler fractions has the form . Hence,

The resulting integral is the integral of the simplest fraction of the third type. Looking ahead a little, we note that you can take it by subsuming it under the differential sign.

Because , That . That's why

Hence,

Now let's move on to describing methods for integrating simple fractions of each of the four types.

Integration of simple fractions of the first type

The direct integration method is ideal for solving this problem:

Example.

Find the set of antiderivatives of a function

Solution.

Let's find the indefinite integral using the properties of the antiderivative, the table of antiderivatives and the integration rule.

Top of page

Integration of simple fractions of the second type

The direct integration method is also suitable for solving this problem:

Example.

Solution.

Top of page

Integration of simple fractions of the third type

First we present the indefinite integral as a sum:

We take the first integral by subsuming it under the differential sign:

That's why,

Let us transform the denominator of the resulting integral:

Hence,

The formula for integrating simple fractions of the third type takes the form:

Example.

Find the indefinite integral .

Solution.

We use the resulting formula:

If we didn’t have this formula, what would we do:

Top of page

Integration of simple fractions of the fourth type

The first step is to put it under the differential sign:

The second step is to find an integral of the form . Integrals of this type are found using recurrence formulas. (See section on integration using recurrence formulas.) The following recurrent formula is suitable for our case:

Example.

Find the indefinite integral

Solution.

For this type of integrand we use the substitution method. Let's introduce a new variable (see the section on integration of irrational functions):

After substitution we have:

We came to finding the integral of a fraction of the fourth type. In our case we have coefficients M = 0, p = 0, q = 1, N = 1 And n=3. We apply the recurrent formula:

After reverse replacement we get the result:

Integrating trigonometric functions

1.Integrals of the form are calculated by transforming the product of trigonometric functions into a sum using the formulas: For example, 2.Integrals of the form , Where m or n– an odd positive number, calculated by subsuming it under the differential sign. For example, 3.Integrals of the form , Where m And n–even positive numbers are calculated using formulas for reducing the degree: For example, 4.Integrals where are calculated by changing the variable: or For example, 5. Integrals of the form are reduced to integrals of rational fractions using a universal trigonometric substitution then (since =[after dividing the numerator and denominator by ]= ; For example, It should be noted that the use of universal substitution often leads to cumbersome calculations.

§5. Integration of the simplest irrationalities

Let's consider methods for integrating the simplest types of irrationality. 1. Functions of this type are integrated in the same way as the simplest rational fractions of the 3rd type: in the denominator, a complete square is isolated from the square trinomial and a new variable is introduced. Example. 2. (under the integral sign – rational function of arguments). Integrals of this type are calculated using substitution. In particular, in integrals of the form we denote . If the integrand contains roots of different degrees: , then denote where n– least common multiple of numbers m,k. Example 1. Example 2. -improper rational fraction, select the whole part: – – – 3.Integrals of the form are calculated using trigonometric substitutions:

44

45 Definite integral

Definite integral- additive monotone normalized functional defined on a set of pairs, the first component of which is an integrable function or functional, and the second is a domain in the set of specifying this function (functional).

Definition

Let it be defined on . Let's divide it into parts with several arbitrary points. Then they say that the segment has been partitioned. Next, choose an arbitrary point , ,

A definite integral of a function on an interval is the limit of integral sums as the rank of the partition tends to zero, if it exists independently of the partition and choice of points, that is

If the specified limit exists, then the function is said to be Riemann integrable.

Designations

· - lower limit.

· - upper limit.

· - integrand function.

· - length of the partial segment.

· - integral sum of the function on the corresponding partition.

· - maximum length of a partial segment.

Properties

If a function is Riemann integrable on , then it is bounded on it.

Geometric meaning

Definite integral as the area of ​​a figure

The definite integral is numerically equal to the area of ​​the figure bounded by the abscissa axis, straight lines and the graph of the function.

Newton-Leibniz theorem

[edit]

(redirected from "Newton-Leibniz Formula")

Newton-Leibniz formula or main theorem of analysis gives the relationship between two operations: taking a definite integral and calculating the antiderivative.

Proof

Let an integrable function be given on an interval. Let's start by noting that

that is, it does not matter which letter (or) is under the sign in the definite integral over the segment.

Let's set an arbitrary value and define a new function . It is defined for all values ​​of , because we know that if there is an integral of on , then there is also an integral of on , where . Let us recall that we consider by definition

(1)

Note that

Let us show that it is continuous on the interval . In fact, let ; Then

and if , then

Thus, it is continuous regardless of whether it has or does not have discontinuities; it is important that it is integrable on .

The figure shows a graph. The area of ​​the variable figure is . Its increment is equal to the area of ​​the figure , which, due to its boundedness, obviously tends to zero at, regardless of whether it is a point of continuity or discontinuity, for example a point.

Let now the function not only be integrable on , but continuous at the point . Let us prove that then the derivative at this point is equal to

(2)

In fact, for the indicated point

(1) , (3)

We put , and since it is constant relative to ,TO . Further, due to continuity at a point, for any one can specify such that for .

which proves that the left-hand side of this inequality is o(1) for .

Passing to the limit in (3) at shows the existence of the derivative of at the point and the validity of equality (2). When we are talking here about the right and left derivatives, respectively.

If a function is continuous on , then, based on what was proven above, the corresponding function

(4)

has a derivative equal to . Therefore, the function is an antiderivative for .

This conclusion is sometimes called the variable upper bound integral theorem or Barrow's theorem.

We have proven that an arbitrary function continuous on an interval has an antiderivative on this interval defined by equality (4). This proves the existence of an antiderivative for any function continuous on an interval.

Let now there be an arbitrary antiderivative of a function on . We know that , where is some constant. Assuming in this equality and taking into account that , we obtain .

Thus, . But

Improper integral

[edit]

Material from Wikipedia - the free encyclopedia

Definite integral called not your own, if at least one of the following conditions is met:

· Limit a or b (or both limits) are infinite;

· The function f(x) has one or more breakpoints inside the segment.

[edit]Improper integrals of the first kind

. Then:

1. If and the integral is called . In this case is called convergent.

, or simply divergent.

Let be defined and continuous on the set from and . Then:

1. If , then the notation is used and the integral is called improper Riemann integral of the first kind. In this case is called convergent.

2. If there is no finite ( or ), then the integral is said to diverge to , or simply divergent.

If a function is defined and continuous on the entire number line, then there may be an improper integral of this function with two infinite limits of integration, defined by the formula:

, where c is an arbitrary number.

[edit] Geometric meaning of an improper integral of the first kind

The improper integral expresses the area of ​​an infinitely long curved trapezoid.

[edit] Examples

[edit]Improper integrals of the second kind

Let it be defined on , suffer an infinite discontinuity at the point x=a and . Then:

1. If , then the notation is used and the integral is called

called divergent to , or simply divergent.

Let it be defined on , suffer an infinite discontinuity at x=b and . Then:

1. If , then the notation is used and the integral is called improper Riemann integral of the second kind. In this case, the integral is called convergent.

2. If or , then the designation remains the same, and called divergent to , or simply divergent.

If the function suffers a discontinuity at an interior point of the segment , then the improper integral of the second kind is determined by the formula:

[edit] Geometric meaning of improper integrals of the second kind

The improper integral expresses the area of ​​an infinitely tall curved trapezoid

[edit] Example

[edit]Isolated case

Let the function be defined on the entire number line and have a discontinuity at the points.

Then we can find the improper integral

[edit] Cauchy criterion

1. Let it be defined on a set from and .

Then converges

2. Let be defined on and .

Then converges

[edit]Absolute convergence

Integral called absolutely convergent, If converges.
If the integral converges absolutely, then it converges.

[edit]Conditional convergence

The integral is called conditionally convergent, if it converges, but diverges.

48 12. Improper integrals.

When considering definite integrals, we assumed that the domain of integration is limited (more specifically, it is the segment [ a ,b ]); For the existence of a definite integral, the integrand must be bounded on [ a ,b ]. We will call definite integrals for which both of these conditions are satisfied (the boundedness of both the domain of integration and the integrand) own; integrals for which these requirements are violated (i.e., either the integrand or the domain of integration is unlimited, or both together) not your own. In this section we will study improper integrals.

• 12.1. Improper integrals over an unbounded interval (improper integrals of the first kind).
• 12.1.1. Definition of an improper integral over an infinite interval. Examples.
• 12.1.2. Newton-Leibniz formula for an improper integral.
• 12.1.3. Comparison criteria for non-negative functions.
• 12.1.3.1. Sign of comparison.
• 12.1.3.2. A sign of comparison in its extreme form.
• 12.1.4. Absolute convergence of improper integrals over an infinite interval.
• 12.1.5. Tests for Abel and Dirichlet convergence.
• 12.2. Improper integrals of unbounded functions (improper integrals of the second kind).
• 12.2.1. Definition of an improper integral of an unbounded function.
• 12.2.1.1. The singularity is at the left end of the integration interval.
• 12.2.1.2. Application of the Newton-Leibniz formula.
• 12.2.1.3. The singularity at the right end of the integration interval.
• 12.2.1.4. Singularity at the inner point of the integration interval.
• 12.2.1.5. Several features on the integration interval.
• 12.2.2. Comparison criteria for non-negative functions.
• 12.2.2.1. Sign of comparison.
• 12.2.2.2. A sign of comparison in its extreme form.
• 12.2.3. Absolute and conditional convergence of improper integrals of discontinuous functions.
• 12.2.4. Tests for Abel and Dirichlet convergence.

12.1. Improper integrals over an unbounded interval

(improper integrals of the first kind).

12.1.1. Definition of an improper integral over an infinite interval. Let the function f (x ) is defined on the semi-axis and is integrable over any interval [ from, implying in each of these cases the existence and finitude of the corresponding limits. Now the solutions to the examples look simpler: .

12.1.3. Comparison criteria for non-negative functions. In this section we will assume that all integrands are non-negative over the entire domain of definition. Until now, we have determined the convergence of the integral by calculating it: if there is a finite limit of the antiderivative with the corresponding tendency ( or ), then the integral converges, otherwise it diverges. When solving practical problems, however, it is important to first establish the fact of convergence itself, and only then calculate the integral (besides, the antiderivative is often not expressed in terms of elementary functions). Let us formulate and prove a number of theorems that allow us to establish the convergence and divergence of improper integrals of nonnegative functions without calculating them.
12.1.3.1. Comparison sign. Let the functions f (x ) And g (x ) integral

TOPIC: Integration of rational fractions.

Attention! When studying one of the basic methods of integration: the integration of rational fractions, it is required to consider polynomials in the complex domain to carry out rigorous proofs. Therefore it is necessary study in advance some properties of complex numbers and operations on them.

Integration of simple rational fractions.

If P(z) And Q(z) are polynomials in the complex domain, then they are rational fractions. It's called correct, if degree P(z) less degree Q(z) , And wrong, if degree R no less than a degree Q.

Any improper fraction can be represented as: ,

P(z) = Q(z) S(z) + R(z),

a R(z) – polynomial whose degree is less than the degree Q(z).

Thus, the integration of rational fractions comes down to the integration of polynomials, that is, power functions, and proper fractions, since it is a proper fraction.

Definition 5. The simplest (or elementary) fractions are the following types of fractions:

1) , 2) , 3) , 4) .

Let's find out how they integrate.

3) (studied earlier).

Theorem 5. Every proper fraction can be represented as a sum of simple fractions (without proof).

Corollary 1. If is a proper rational fraction, and if among the roots of the polynomial there are only simple real roots, then in the decomposition of the fraction into the sum of simple fractions there will be only simple fractions of the 1st type:

Example 1.

Corollary 2. If is a proper rational fraction, and if among the roots of the polynomial there are only multiple real roots, then in the decomposition of the fraction into the sum of simple fractions there will be only simple fractions of the 1st and 2nd types:

Example 2.

Corollary 3. If is a proper rational fraction, and if among the roots of the polynomial there are only simple complex conjugate roots, then in the decomposition of the fraction into the sum of simple fractions there will be only simple fractions of the 3rd type:

Example 3.

Corollary 4. If is a proper rational fraction, and if among the roots of the polynomial there are only multiple complex conjugate roots, then in the decomposition of the fraction into the sum of simple fractions there will be only simple fractions of the 3rd and 4th types:

To determine the unknown coefficients in the given expansions proceed as follows. The left and right sides of the expansion containing unknown coefficients are multiplied by The equality of two polynomials is obtained. From it, equations for the required coefficients are obtained using the following:

1. equality is true for any values ​​of X (partial value method). In this case, any number of equations are obtained, any m of which allow one to find the unknown coefficients.

2. the coefficients coincide for the same degrees of X (method of indefinite coefficients). In this case, a system of m - equations with m - unknowns is obtained, from which the unknown coefficients are found.

3. combined method.

Example 5. Expand a fraction to the simplest.

Solution:

Let's find the coefficients A and B.

Method 1 - private value method:

Method 2 – method of undetermined coefficients:

Answer:

Integrating rational fractions.

Theorem 6. The indefinite integral of any rational fraction on any interval on which its denominator is not equal to zero exists and is expressed through elementary functions, namely rational fractions, logarithms and arctangents.

Proof.

Let's imagine a rational fraction in the form: . In this case, the last term is a proper fraction, and according to Theorem 5 it can be represented as a linear combination of simple fractions. Thus, the integration of a rational fraction is reduced to the integration of a polynomial S(x) and simple fractions, the antiderivatives of which, as has been shown, have the form indicated in the theorem.

Comment. The main difficulty in this case is the factorization of the denominator, that is, the search for all its roots.

Example 1. Find the integral

The integrand is a proper rational fraction. The expansion of the denominator into irreducible factors has the form This means that the expansion of the integrand into a sum of simple fractions has the following form:

Let's find the expansion coefficients using a combined method:

Thus,

Example 2. Find the integral

The integrand is an improper fraction, so we isolate the whole part:

The first of the integrals is tabular, and we calculate the second by decomposing the proper fraction into simple ones:

Using the method of undetermined coefficients, we have:

Thus,

Enter the function for which you need to find the integral

After calculating the indefinite integral, you will be able to receive a free DETAILED solution to the integral you entered.

Let's find the solution to the indefinite integral of the function f(x) (the antiderivative of the function).

Examples

Using degree
(square and cube) and fractions

(x^2 - 1)/(x^3 + 1)

Square root

Sqrt(x)/(x + 1)

Cube root

Cbrt(x)/(3*x + 2)

Using sine and cosine

2*sin(x)*cos(x)

arcsine

X*arcsin(x)

arc cosine

X*arccos(x)

Application of the logarithm

X*log(x, 10)

Natural logarithm

Exhibitor

Tg(x)*sin(x)

Cotangent

Ctg(x)*cos(x)

Irrational fractions

(sqrt(x) - 1)/sqrt(x^2 - x - 1)

Arctangent

X*arctg(x)

Arccotangent

X*arсctg(x)

Hyperbolic sine and cosine

2*sh(x)*ch(x)

Hyperbolic tangent and cotangent

Ctgh(x)/tgh(x)

Hyperbolic arcsine and arccosine

X^2*arcsinh(x)*arccosh(x)

Hyberbolic arctangent and arccotangent

X^2*arctgh(x)*arcctgh(x)

Rules for entering expressions and functions

Expressions can consist of functions (notations are given in alphabetical order): absolute(x) Absolute value x
(module x or |x|) arccos(x) Function - arc cosine of x arccosh(x) Arc cosine hyperbolic from x arcsin(x) Arcsine from x arcsinh(x) Arcsine hyperbolic from x arctan(x) Function - arctangent of x arctgh(x) Arctangent hyperbolic from x e e a number that is approximately equal to 2.7 exp(x) Function - exponent of x(as e^x) log(x) or ln(x) Natural logarithm of x
(To get log7(x), you need to enter log(x)/log(7) (or, for example, for log10(x)=log(x)/log(10)) pi The number is "Pi", which is approximately equal to 3.14 sin(x) Function - Sine of x cos(x) Function - Cosine of x sinh(x) Function - Sine hyperbolic from x cosh(x) Function - Cosine hyperbolic from x sqrt(x) Function - square root from x sqr(x) or x^2 Function - Square x tan(x) Function - Tangent from x tgh(x) Function - Tangent hyperbolic from x cbrt(x) Function - cube root of x

The following operations can be used in expressions: Real numbers enter as 7.5 , Not 7,5 2*x- multiplication 3/x- division x^3- exponentiation x+7- addition x - 6- subtraction
Other features: floor(x) Function - rounding x downward (example floor(4.5)==4.0) ceiling(x) Function - rounding x upward (example ceiling(4.5)==5.0) sign(x) Function - Sign x erf(x) Error function (or probability integral) laplace(x) Laplace function

Integration of a fractional-rational function.
Uncertain coefficient method

We continue to work on integrating fractions. We have already looked at integrals of some types of fractions in the lesson, and this lesson in some sense can be considered a continuation. To successfully understand the material, basic integration skills are required, so if you have just started studying integrals, that is, you are a beginner, then you need to start with the article Indefinite integral. Examples of solutions.

Oddly enough, now we will be engaged not so much in finding integrals, but... in solving systems of linear equations. In this regard urgently I recommend attending the lesson. Namely, you need to be well versed in substitution methods (“the school” method and the method of term-by-term addition (subtraction) of system equations).

What is a fractional rational function? In simple words, a fractional-rational function is a fraction whose numerator and denominator contain polynomials or products of polynomials. Moreover, the fractions are more sophisticated than those discussed in the article Integrating Some Fractions.

Integrating a Proper Fractional-Rational Function

Immediately an example and a typical algorithm for solving the integral of a fractional-rational function.

Example 1

Step 1. The first thing we ALWAYS do when solving an integral of a fractional rational function is to clarify the following question: is the fraction proper? This step is performed verbally, and now I will explain how:

First we look at the numerator and find out senior degree polynomial:

The leading power of the numerator is two.

Now we look at the denominator and find out senior degree denominator. The obvious way is to open the brackets and bring similar terms, but you can do it simpler, in each find the highest degree in brackets

and mentally multiply: - thus, the highest degree of the denominator is equal to three. It is quite obvious that if we actually open the brackets, we will not get a degree greater than three.

Conclusion: Major degree of numerator STRICTLY is less than the highest power of the denominator, which means the fraction is proper.

If in this example the numerator contained the polynomial 3, 4, 5, etc. degrees, then the fraction would be wrong.

Now we will consider only the correct fractional rational functions. We will examine the case when the degree of the numerator is greater than or equal to the degree of the denominator at the end of the lesson.

Step 2. Let's factorize the denominator. Let's look at our denominator:

Generally speaking, this is already a product of factors, but, nevertheless, we ask ourselves: is it possible to expand something else? The object of torture will undoubtedly be the square trinomial. Solving the quadratic equation:

The discriminant is greater than zero, which means that the trinomial really can be factorized:

General rule: EVERYTHING that CAN be factored in the denominator - we factor it

Let's begin to formulate a solution:

Step 3. Using the method of indefinite coefficients, we expand the integrand into a sum of simple (elementary) fractions. Now it will be clearer.

Let's look at our integrand function:

And, you know, somehow an intuitive thought pops up that it would be nice to turn our large fraction into several small ones. For example, like this:

The question arises, is it even possible to do this? Let us breathe a sigh of relief, the corresponding theorem of mathematical analysis states – IT IS POSSIBLE. Such a decomposition exists and is unique.

There's just one catch, the odds are Bye We don’t know, hence the name – the method of indefinite coefficients.

As you guessed, subsequent body movements are like that, don’t cackle! will be aimed at just RECOGNIZING them - to find out what they are equal to.

Be careful, I will explain in detail only once!

So, let's start dancing from:

On the left side we reduce the expression to a common denominator:

Now we can safely get rid of the denominators (since they are the same):

On the left side we open the brackets, but do not touch the unknown coefficients for now:

At the same time, we repeat the school rule for multiplying polynomials. When I was a teacher, I learned to pronounce this rule with a straight face: In order to multiply a polynomial by a polynomial, you need to multiply each term of one polynomial by each term of the other polynomial.

From the point of view of a clear explanation, it is better to put the coefficients in brackets (although I personally never do this in order to save time):

We compose a system of linear equations.
First we look for senior degrees:

And we write the corresponding coefficients into the first equation of the system:

Remember the following point well. What would happen if there were no s on the right side at all? Let's say, would it just show off without any square? In this case, in the equation of the system it would be necessary to put a zero on the right: . Why zero? But because on the right side you can always assign this same square with zero: If on the right side there are no variables and/or a free term, then we put zeros on the right sides of the corresponding equations of the system.

We write the corresponding coefficients into the second equation of the system:

And finally, mineral water, we select free members.

Eh...I was kind of joking. Jokes aside - mathematics is a serious science. In our institute group, no one laughed when the assistant professor said that she would scatter the terms along the number line and choose the largest ones. Let's get serious. Although... whoever lives to see the end of this lesson will still smile quietly.

The system is ready:

We solve the system:

(1) From the first equation we express and substitute it into the 2nd and 3rd equations of the system. In fact, it was possible to express (or another letter) from another equation, but in this case it is advantageous to express it from the 1st equation, since there the smallest odds.

(2) We present similar terms in the 2nd and 3rd equations.

(3) We add the 2nd and 3rd equations term by term, obtaining the equality , from which it follows that

(4) We substitute into the second (or third) equation, from where we find that

(5) Substitute and into the first equation, obtaining .

If you have any difficulties with the methods of solving the system, practice them in class. How to solve a system of linear equations?

After solving the system, it is always useful to check - substitute the found values every equation of the system, as a result everything should “converge”.

Almost there. The coefficients were found, and:

The finished job should look something like this:

As you can see, the main difficulty of the task was to compose (correctly!) and solve (correctly!) a system of linear equations. And at the final stage, everything is not so difficult: we use the linearity properties of the indefinite integral and integrate. Please note that under each of the three integrals we have a “free” complex function; I talked about the features of its integration in the lesson Variable change method in indefinite integral.

Check: Differentiate the answer:

The original integrand function has been obtained, which means that the integral has been found correctly.
During the verification, we had to reduce the expression to a common denominator, and this is not accidental. The method of indefinite coefficients and reducing an expression to a common denominator are mutually inverse actions.

Example 2

Find the indefinite integral.

Let's return to the fraction from the first example: . It is easy to notice that in the denominator all the factors are DIFFERENT. The question arises, what to do if, for example, the following fraction is given: ? Here we have degrees in the denominator, or, mathematically, multiples. In addition, there is a quadratic trinomial that cannot be factorized (it is easy to verify that the discriminant of the equation is negative, so the trinomial cannot be factorized). What to do? The expansion into a sum of elementary fractions will look something like with unknown coefficients at the top or something else?

Example 3

Introduce a function

Step 1. Checking if we have a proper fraction
Major numerator: 2
Highest degree of denominator: 8
, which means the fraction is correct.

Step 2. Is it possible to factor something in the denominator? Obviously not, everything is already laid out. The square trinomial cannot be expanded into a product for the reasons stated above. Hood. Less work.

Step 3. Let's imagine a fractional-rational function as a sum of elementary fractions.
In this case, the expansion has the following form:

Let's look at our denominator:
When decomposing a fractional-rational function into a sum of elementary fractions, three fundamental points can be distinguished:

1) If the denominator contains a “lonely” factor to the first power (in our case), then we put an indefinite coefficient at the top (in our case). Examples No. 1, 2 consisted only of such “lonely” factors.

2) If the denominator has multiple multiplier, then you need to decompose it like this:
- that is, sequentially go through all the degrees of “X” from the first to the nth degree. In our example there are two multiple factors: and , take another look at the expansion I gave and make sure that they are expanded exactly according to this rule.

3) If the denominator contains an indecomposable polynomial of the second degree (in our case), then when decomposing in the numerator you need to write a linear function with indefinite coefficients (in our case with indefinite coefficients and ).

In fact, there is another 4th case, but I will keep silent about it, since in practice it is extremely rare.

Example 4

Introduce a function as a sum of elementary fractions with unknown coefficients.

This is an example for independent decision. Full solution and answer at the end of the lesson.
Follow the algorithm strictly!

If you understand the principles by which you need to expand a fractional-rational function into a sum, you can chew through almost any integral of the type under consideration.

Example 5

Find the indefinite integral.

Step 1. Obviously the fraction is correct:

Step 2. Is it possible to factor something in the denominator? Can. Here is the sum of cubes . Factor the denominator using the abbreviated multiplication formula

Step 3. Using the method of indefinite coefficients, we expand the integrand into a sum of elementary fractions:

Please note that the polynomial cannot be factorized (check that the discriminant is negative), so at the top we put a linear function with unknown coefficients, and not just one letter.

We bring the fraction to a common denominator:

Let's compose and solve the system:

(1) We express from the first equation and substitute it into the second equation of the system (this is the most rational way).

(2) We present similar terms in the second equation.

(3) We add the second and third equations of the system term by term.

All further calculations are, in principle, oral, since the system is simple.

(1) We write down the sum of fractions in accordance with the found coefficients.

(2) We use the linearity properties of the indefinite integral. What happened in the second integral? You can familiarize yourself with this method in the last paragraph of the lesson. Integrating Some Fractions.

(3) Once again we use the properties of linearity. In the third integral we begin to isolate the complete square (penultimate paragraph of the lesson Integrating Some Fractions).

(4) We take the second integral, in the third we select the complete square.

(5) Take the third integral. Ready.