How do you integrate f(x)=(x-2)/((x^2-3)(x-3)(x-1))f(x)=x2(x23)(x3)(x1) using partial fractions?

1 Answer
Jan 5, 2016

int f(x)dx=1/12ln(x+sqrt3)+1/12ln(x-sqrt3)+1/12ln(x-3)-1/4ln(x-1)+Cf(x)dx=112ln(x+3)+112ln(x3)+112ln(x3)14ln(x1)+C

Explanation:

First we need to decompose this function to a sum of partial fractions (we assume it can be done):

(x-2)/((x+sqrt3)(x-sqrt3)(x-3)(x-1))=color(red)A/(x+sqrt3)+color(green)B/(x-sqrt3)+color(blue)C/(x-3)+color(orange)D/(x-1)x2(x+3)(x3)(x3)(x1)=Ax+3+Bx3+Cx3+Dx1

but after bringing it to common denominator, the numerator equals
color(red)A(x-sqrt3)(x-3)(x-1)+(x+sqrt3)color(green)B(x-3)(x-1)+(x+sqrt3)(x-sqrt3)color(blue)C(x-1)+(x+sqrt3)(x-sqrt3)(x-3)color(orange)DA(x3)(x3)(x1)+(x+3)B(x3)(x1)+(x+3)(x3)C(x1)+(x+3)(x3)(x3)D

Using Vieta's formulas for 3rd degree polynomials, we get
color(red)A(x^3-(4+sqrt3)x^2+(3+4sqrt3)x-3sqrt3) +color(green)B(x^3-(4-sqrt3)x^2+(3-4sqrt3)x+3sqrt3) +color(blue)C(x^3-x^2-3x+3)+color(orange)D(x^3-3x^2-3x+9)A(x3(4+3)x2+(3+43)x33)+B(x3(43)x2+(343)x+33)+C(x3x23x+3)+D(x33x23x+9)

Combining like terms:
(color(red)A+color(green)B+color(blue)C+color(orange)D)x^3
-((4+sqrt3)color(red)A+(4-sqrt3)color(green)B+color(blue)C +3color(orange)D)x^2 +((3+4sqrt3)color(red)A+(3-4sqrt3)color(green)B-3color(blue)C-3color(orange)D)x +(-3sqrt3color(red)A+3sqrt3color(green)B+3color(blue)C +9color(orange)D)

but we remember it's equal to x-2, so we have
{(color(red)A+color(green)B+color(blue)C+color(orange)D=0), ((4+sqrt3)color(red)A+(4-sqrt3)color(green)B+color(blue)C +3color(orange)D=0), ((3+4sqrt3)color(red)A+(3-4sqrt3)color(green)B-3color(blue)C-3color(orange)D=1), (-3sqrt3color(red)A+3sqrt3color(green)B+3color(blue)C +9color(orange)D=-2):}

Solving the system gives:
color(red)A=1/12, color(green)B=1/12, color(blue)C=1/12, color(orange)D=-1/4

So we have:
f(x)=(1/12)/(x+sqrt3)+(1/12)/(x-sqrt3)+(1/12)/(x-3)-(1/4)/(x-1)

Integral of sum is sum of integrals, so here's integral of the first term:
int(1/12)/(x+sqrt3)dx=1/12int1/(x+sqrt3)dx

substituting u=x+sqrt3 and du=dx we have
1/12int1/udu=1/12lnu+C=1/12ln(x+sqrt3)+C
The other 3 terms are done similarly.