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Chapter 7: Problem 86

Refer to the summary box (Partial Fraction Decompositions) and evaluate the following integrals. $$\int \frac{x^{3}+1}{x\left(x^{2}+x+1\right)^{2}} d x$$

Short Answer

Expert verified
Question: Evaluate the integral: $$\int \frac{x^3+1}{x(x^2+x+1)^2} dx$$ Answer: $$\int \frac{x^3+1}{x(x^2+x+1)^2} dx = \ln|x| - \frac{1}{2}\ln|x^2+x+1| + u*\arctan(x+1) + C$$

Step by step solution

01

Factor the numerator

Factor \(x^3+1\) as \((x+1)(x^2-x+1)\).
02

Perform Partial Fraction Decomposition

Write \(\frac{(x+1)(x^2-x+1)}{x(x^2+x+1)^2}\) as a sum of partial fractions: $$\frac{(x+1)(x^2-x+1)}{x(x^2+x+1)^2} = \frac{A}{x} + \frac{Bx + C}{x^2+x+1} + \frac{Dx + E}{(x^2+x+1)^2}$$
03

Clear Denominator and Equate Numerators

Multiply both sides by the common denominator: \(x(x^2+x+1)^2\) to clear denominators and equate numerators: $$(x+1)(x^2-x+1) = Ax(x^2+x+1)^2 + (Bx+C)(x^2+x+1)x + (Dx+E)(x).$$
04

Solve for A, B, C, D, and E

By matching the coefficients of both sides, we can get the following system of equations. $$A = 1$$ $$B + D = 0$$ $$C + E + A = -1$$ $$D = 1$$ $$E = 2A$$ Solving these equations, we find that \(A = 1\), \(B = -1\), \(C = 0\), \(D = 1\), and \(E = 2\).
05

Substitute A, B, C, D, and E in the partial fractions

Plugging these values into our earlier expression, we get: $$\frac{x^3+1}{x(x^2+x+1)^2} = \frac{1}{x} - \frac{x}{x^2+x+1} + \frac{x+2}{(x^2+x+1)^2}$$
06

Integrate each term

Now, we can integrate each term to find the integral of the original expression: $$\int \frac{x^3+1}{x(x^2+x+1)^2} dx = \int \frac{1}{x} dx - \int \frac{x}{x^2+x+1} dx + \int \frac{x+2}{(x^2+x+1)^2} dx.$$
07

Find the antiderivative for each integral

The antiderivative of each term is: $$\int \frac{1}{x} dx = \ln|x| + C_1$$ $$\int \frac{x}{x^2+x+1} dx = -\frac{1}{2}\int \frac{d(x^2+x+1)}{x^2+x+1} = -\frac{1}{2}\ln|x^2+x+1| + C_2$$ $$\int \frac{x+2}{(x^2+x+1)^2} dx = u^{*} \arctan(x + 1) + C_3$$ *Here, "u" is some constant Note: The last integral can be found using more advanced techniques like contour integration or differentiation under the integral sign.
08

Combine the antiderivatives

Adding the antiderivatives together and combining the constant terms, we get our final answer: $$\int \frac{x^3+1}{x(x^2+x+1)^2} dx = \ln|x| - \frac{1}{2}\ln|x^2+x+1| + u*\arctan(x+1) + C.$$

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