Evaluate the following integral:
First we simplify numerator, we get
Now the denominator is factorized, so let separate the fraction through partial fraction, hence let
⇒ 5x + 1 = A(x – 1) + B(x + 2)……(ii)
We need to solve for A and B. One way to do this is to pick values for x which will cancel each variable.
Put x = 1 in the above equation, we get
⇒ 5(1) + 1 = A(1 – 1) + B(1 + 2)
⇒ 6 = 0 + 3B
⇒ B = 2
Now put x = – 2 in equation (ii), we get
⇒ 5( – 2) + 1 = A(( – 2) – 1) + B(( – 2) + 2)
⇒ – 9 = – 3A + 0
⇒ A = 3
We put the values of A and B values back into our partial fractions in equation (i) and replace this as the integrand. We get
Split up the integral,
Let substitute u = x + 2 ⇒ du = dx and z = x – 1 ⇒ dz = dx, so the above equation becomes,
On integrating we get
Substituting back, we get
Note: the absolute value signs account for the domain of the natural log function (x>0).
Hence,