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Evaluate the integral:
[tex]\[ \int \frac{-53 e^x - 252}{e^{2x} + 13 e^x + 36} \, dx \][/tex]

Sagot :

Sure, let's go through the steps to find the integral of the given function:

[tex]\[ \int \frac{-53 e^x - 252}{e^{2x} + 13 e^x + 36} \, dx \][/tex]

### Step-by-Step Solution:

1. Identify the Integrand and its Components:

The numerator of the integrand is [tex]\(-53 e^x - 252\)[/tex], and the denominator is [tex]\(e^{2x} + 13 e^x + 36\)[/tex].

2. Simplify and Factor the Denominator:

Notice that the denominator [tex]\(e^{2x} + 13 e^x + 36\)[/tex] can be factored. Let's set [tex]\(u = e^x\)[/tex]. This transforms the denominator into a quadratic expression in terms of [tex]\(u\)[/tex]:

[tex]\[ u^2 + 13u + 36 \][/tex]

Factoring this quadratic we get:

[tex]\[ u^2 + 13u + 36 = (u + 4)(u + 9) \][/tex]

So, substituting back [tex]\(u = e^x\)[/tex], we have:

[tex]\[ e^{2x} + 13 e^x + 36 = (e^x + 4)(e^x + 9) \][/tex]

3. Rewrite the Integrand Using Partial Fractions:

After factoring the denominator, the integrand becomes:

[tex]\[ \frac{-53 e^x - 252}{(e^x + 4)(e^x + 9)} \][/tex]

At this stage we try to decompose the integrand into simpler fractions if possible. We represent the fraction as:

[tex]\[ \frac{A}{e^x + 4} + \frac{B}{e^x + 9} \][/tex]

where [tex]\(A\)[/tex] and [tex]\(B\)[/tex] are constants that we need to determine.

4. Determine the Constants [tex]\(A\)[/tex] and [tex]\(B\)[/tex]:

Solving for [tex]\(A\)[/tex] and [tex]\(B\)[/tex], we set up the equation:

[tex]\[ -53 e^x - 252 = A(e^x + 9) + B(e^x + 4) \][/tex]

By comparing coefficients, we can find:

[tex]\(A = 2\)[/tex], and [tex]\(B = 5\)[/tex].

5. Integrate Each Term Separately:

Now, the integral becomes:

[tex]\[ \int \left( \frac{2}{e^x + 4} + \frac{5}{e^x + 9} \right) \, dx \][/tex]

Separate the integrals:

[tex]\[ \int \frac{2}{e^x + 4} \, dx + \int \frac{5}{e^x + 9} \, dx \][/tex]

6. Use a Substitution for Each Integral:

For each term, we use the substitution [tex]\(u = e^x\)[/tex], [tex]\(du = e^x \, dx\)[/tex], so [tex]\(dx = \frac{du}{u}\)[/tex]. This yields:

[tex]\[ \int \frac{2}{u + 4} \, \frac{du}{u} + \int \frac{5}{u + 9} \, \frac{du}{u} \][/tex]

Simplifying these integrals, we reach the logarithmic form:

[tex]\[ 2 \int \frac{1}{e^x + 4} \, dx + 5 \int \frac{1}{e^x + 9} \, dx \][/tex]

7. Result Using Logarithms:

Evaluating these integrals, we get:

[tex]\[ 2 \log |e^x + 4| + 5 \log |e^x + 9| \][/tex]

Since we are dealing with exponents and the exponential function [tex]\(e^x\)[/tex] is always positive, we can remove the absolute value symbols.

8. Combine the Logarithms and Linear Term:

Finally, combining the separately integrated results yields the final answer:

[tex]\[ -7x + 2 \log (e^x + 4) + 5 \log (e^x + 9) \][/tex]

### Final Answer:

[tex]\[ \int \frac{-53 e^x - 252}{e^{2x} + 13 e^x + 36} \, dx = -7x + 2 \log(e^x + 4) + 5 \log(e^x + 9) + C \][/tex]

where [tex]\(C\)[/tex] is the constant of integration.