How do you find the integral of #(x^3-4x-10)/(x^2-x-6)# from 0 to 1?

1 Answer
Mar 19, 2018

#int_0^1 (x^3-4x-10)/(x^2-x-6)dx = 3/2 + ln( 3/2) #

Explanation:

Solve first the indefinite integral, noting that:

#x^2-x-6 = (x-3)(x+2)#

#x^3-4x-10 = x(x^2-4) -10 = x (x-2) (x+2) -10#

So:

#int (x^3-4x-10)/(x^2-x-6)dx = int (x (x-2) (x+2) -10)/((x-3)(x+2))dx#

Simplify and use the linearity of the integral:

#int (x^3-4x-10)/(x^2-x-6) dx = int (x (x-2) )/(x-3) dx -10int dx/((x-3)(x+2))#

Solve the integrals separately. In the first substitute #(x-3) = u#:

#int (x (x-2) )/(x-3) dx = int ((u+3)(u+1))/udu#

#int (x (x-2) )/(x-3) dx = int (u^2+4u +3)/udu#

and always using linearity:

#int (x (x-2) )/(x-3) dx = int udu +4 int du +3int (du)/u#

#int (x (x-2) )/(x-3) dx = u^2/2 +4 u +3 ln abs u#

and undoing the substitution:

#int (x (x-2) )/(x+3) dx = (x-3)^2/2 +4 (x-3) +3 ln abs (x-3)+C#

Solve the second integral with partial fraction decomposition:

#1/((x-3)(x+2)) = A/(x-3)+B/(x+2)#

#1 = A(x+2) +B(x-3)#

#1 = (A+B)x + (2A -3B)#

#{(A+B=0),(2A-3B=1):}#

#{(A=1/5),(B=-1/5):}#

So:

#int dx/((x-3)(x+2)) = 1/5 int dx/(x-3) -1/5 int dx/(x+2)#

#int dx/((x-3)(x+2)) = 1/5ln abs(x-3) -1/5 ln abs(x+2)+C#

Put together the partial results:

#int (x^3-4x-10)/(x^2-x-6)dx = (x-3)^2/2 +4 (x-3) +3 ln abs (x-3)-2 ln abs(x-3) +2 ln abs(x+2)+C#

Simplifying and absorbing the constants in #C#:

#int (x^3-4x-10)/(x^2-x-6)dx = x^2/2 +x + ln abs (x-3)+2 ln abs(x+2)+C#

Now:

#int_0^1 (x^3-4x-10)/(x^2-x-6)dx = 1/2 +1 +ln 2+2ln 3 - ln3 -2ln2#

#int_0^1 (x^3-4x-10)/(x^2-x-6)dx = 3/2 - ln 2+ln 3 #

#int_0^1 (x^3-4x-10)/(x^2-x-6)dx = 3/2 + ln( 3/2) #