If \(\int e^x\left(\displaystyle\frac{x-1}{(x+1)^3}\right)dx = \displaystyle\frac{A.\, e^x}{(x + 1)^B}+C\), where C is constant of integration then which of the following are correct?
(A) A = −1
(B) A = 1
(C) B = 3
(D) B = 2
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (B) and (D) only
The integrand is put in the form \(e^x[f(x) + f'(x)]\), whose integral is \(e^x f(x) + C\):
\(\displaystyle \frac{x - 1}{(x + 1)^3} = \frac{(x + 1) - 2}{(x + 1)^3} = \frac{1}{(x + 1)^2} - \frac{2}{(x + 1)^3}\)
With \(f(x) = \frac{1}{(x + 1)^2}\):
\(\displaystyle f'(x) = -\frac{2}{(x + 1)^3}\)
So
\(\displaystyle \int e^x\left(\frac{x - 1}{(x + 1)^3}\right)dx = \frac{e^x}{(x + 1)^2} + C\)
So A = 1 and B = 2.
Checking the statements
(A) A = −1 — Incorrect. A is 1.
(B) A = 1 — Correct. The coefficient of \(\frac{e^x}{(x + 1)^2}\) is 1.
(C) B = 3 — Incorrect. B is 2.
(D) B = 2 — Correct. The power of \(x + 1\) is 2.