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Question 26 of 39

Q.For ∫x−1(x+1)3 ex dx=exf(x)+c\displaystyle\int \dfrac{x - 1}{(x + 1)^3}\, e^x\, dx = e^x f(x) + c, f(x)=(x+1)2f(x) = (x + 1)^2.

(a) True
(b) False
Maharashtra MsbshseMaharashtra HSC (MSBSHSE) Board 2024MCQ· 1mImportance★★★★★
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Using ∫ex[f(x)+f′(x)] dx=exf(x)+c\int e^x[f(x)+f'(x)]\,dx = e^x f(x)+c, the integrand rearranges to f(x)=1(x+1)2f(x) = \dfrac{1}{(x+1)^2}, so the claim f(x)=(x+1)2f(x) = (x+1)^2 is false.

Split the rational part by writing x−1=(x+1)−2x - 1 = (x + 1) - 2:

x−1(x+1)3=(x+1)−2(x+1)3=1(x+1)2−2(x+1)3.\frac{x-1}{(x+1)^3} = \frac{(x+1) - 2}{(x+1)^3} = \frac{1}{(x+1)^2} - \frac{2}{(x+1)^3}.

Take f(x)=1(x+1)2=(x+1)−2f(x) = \dfrac{1}{(x+1)^2} = (x+1)^{-2}. Then

f′(x)=−2(x+1)−3=−2(x+1)3.f'(x) = -2(x+1)^{-3} = -\frac{2}{(x+1)^3}.

So f(x)+f′(x)=1(x+1)2−2(x+1)3f(x) + f'(x) = \dfrac{1}{(x+1)^2} - \dfrac{2}{(x+1)^3}, which is exactly the bracketed integrand. Hence …

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