How do you integrate int (x-1)/( x^4 (x-1)^2)x1x4(x1)2 using partial fractions?

2 Answers

Ln(x-1)-Lnx+x^(-1)+1/2*x^(-2)+1/3*x^(-3)+Cln(x1)lnx+x1+12x2+13x3+C

Explanation:

1) I used basic fractions method for integration.

int ((x-1) dx)/[x^4*(x-1)^2](x1)dxx4(x1)2

=int dx/[x^4*(x-1)]=dxx4(x1)

I decomposed integrand into basic fractions,

1/[x^4*(x-1)]=A/(x-1)+B/x+C/x^2+D/x^3+E/x^41x4(x1)=Ax1+Bx+Cx2+Dx3+Ex4

A*x^4+B*x^3*(x-1)+C*x^2*(x-1)+D*x*(x-1)+E*(x-1)=1Ax4+Bx3(x1)+Cx2(x1)+Dx(x1)+E(x1)=1

Set x=0x=0, -E=-1E=1 or E=-1E=1

Set x=1x=1, A=1A=1

I took differentiaton both sides,

4A*x^3+B*(4x^3-3x^2)+C*(3x^2-2x)+D*(2x-1)+E=04Ax3+B(4x33x2)+C(3x22x)+D(2x1)+E=0

Set x=0x=0, -D+E=0D+E=0 or D=E=-1D=E=1

I took differentiaton both sides,

12A*x^2+B*(12x^2-6x)+C*(6x-2)+2D=012Ax2+B(12x26x)+C(6x2)+2D=0

Set x=0x=0, -2C+2D=02C+2D=0 or C=D=-1C=D=1

I took differentiaton both sides,

24A*x+B*(24x-6)+6C=024Ax+B(24x6)+6C=0

Set x=0x=0, -6B+6C=06B+6C=0 or B=C=-1B=C=1

Thus,

int dx/[x^4*(x-1)]dxx4(x1)

=int dx/(x-1)dxx1-int dx/xdxx-int dx/x^2dxx2-int dx/x^3dxx3-int dx/x^4dxx4

=Ln(x-1)-Lnx+x^(-1)+1/2*x^(-2)+1/3*x^(-3)+Cln(x1)lnx+x1+12x2+13x3+C

Sep 9, 2017

How about NOT using partial fractions and instead using the binomial theorem (or the sum of geometrical series, same thing) after cancelling the x-1x1?

Explanation:

int 1/(x^4(x-1))dx1x4(x1)dx
= -int 1/(x^4(1-x))dx=1x4(1x)dx
=-int(1/x^4)(1+x+x^2+x^3+x^4...)dx (for suitable x, sum to infinity of GP with common ratio x, backwards, or binomial expansion with n=-1)
=-intx^-4+x^-3+x^-2+x^-1+1+ x+x^2+x^3+...dx
=1/3x^3+1/2x^-2+x^-1-ln|x|-int1/(1-x) dx (sum to infinity of GP again, forwards)
=1/3x^3+1/2x^-2+x^-1-ln|x|+ln|x-1|+c

The binomial expansion above works only for |x|<1. To deal with |x|>1:
int 1/(x^4(x-1))dx
=int 1/(x^5(1-1/x))dx
Using the Binomial Theorem, or sum to infinity of GP with first term 1 and common ratio x^-1:
=intx^-5(1+x^-1+x^-2+x^-3...)dx provided |x|>1
=int x^-5+x^-6+x^-7... dx
=int x^-1+x^-2+x^-3+x^-4+x^-5... dx -
\ \ \ \ \ int x^-1+x^-2+x^-3+x^-4dx

Using sum to infinity of GP or binomial theorem:
=intx^-1/(1-x^-1)dx - int x^-1+x^-2+x^-3+x^-4dx
=int 1/(x-1)dx - int x^-1+x^-2+x^-3+x^-4dx
=ln|x-1|-ln|x| + 1/x+1/2x^2+1/3x^3+c
which is the same as the result for |x|<1. Good!