Thursday, October 11, 2012

What is indefinite integral of f(x)=(x^5+2)/(x^2-1)?

We notice that the degree of the numerator is higher than
the degree of the denominator, so we'll perform the long
division.


x^5+2 = (x^2 - 1)(x^3 + x) + x +
2


We'll divide by x^2 -
1:


(x^5+2)/(x^2 - 1) = x^3 + x + ( x + 2)/(x^2 -
1)


We'll integrate both
sides:


Int (x^5+2)dx/(x^2 - 1) = Int x^3dx + Int xdx +
Int( x + 2)dx/(x^2 - 1)


We'll calculate Int( x + 2)dx/(x^2
- 1).


We'll decompose the fraction ( x + 2)/(x^2 - 1)  into
elementary fractions:


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


( x + 2)/( x- 1)(x+1) = A/(x-1) +
B/(x+1)


x + 2 = Ax + A + Bx -
B


x + 2 = x(A + B) + A - B


A+B
= 1 (1)


A-B=2 (2)


We'll add
(1) and (2):


2A =3


A =
3/2


B = -1/2


Int( x +
2)dx/(x^2 - 1) = (3/2)Int dx/(x-1) - (1/2)Int
dx/(x+1)


Int( x + 2)dx/(x^2 - 1) = (3/2)ln|x-1| -
(1/2)ln|x+1|


Int (x^5+2)dx/(x^2 - 1) = x^4/4
+ x^2/2 +  (3/2)ln|x-1| - (1/2)ln|x+1| + C

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