Question

Decompose -6x/(x+2)(x+8) into partial fractions.

Answer 1

The partial fraction expansion takes the form

-6x/((x + 2) (x + 8)) = a/(x + 2) + b/(x + 8)

Both factors in the denominator are linear, so the numerators in the corresponding partial fractions have degree 1 - 1 = 0 and are thus constants.

Combine the fractions on the right side into one with a common denominator, then set the numerators on both sides of the equation equal to each other:

-6x = a (x + 8) + b (x + 2)

Expand the right side and collect terms by powers of x :

-6x = (a + b) x + (8a + 2b)

It follows that

a + b = -6   and   8a + 2b = 0

==>   a = -2   and   b = 8

So we end up with

-6x/((x + 2) (x + 8)) = -2/(x + 2) + 8/(x + 8)