Think about e x e^x ?

Calculus Level 2

x e x ( 1 + x ) 2 d x = ? \large \displaystyle \int \dfrac{xe^x}{(1+x)^2} \, dx = \, ?

Clarification : C C denotes the arbitrary constant of integration .

e x x + 1 + C \large \dfrac{e^x}{x+1}+C e x 1 x + 1 + C \large e^x-\dfrac{1}{x+1}+C e x ( x + 1 ) 2 + C \large \dfrac{e^x}{(x+1)^2}+C x e x + C \large xe^x+C

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3 solutions

Sparsh Sarode
Jun 4, 2016

\large \displaystyle \int ( x + 1 1 ) e x ( 1 + x ) 2 d x \dfrac{(x+1-1)e^x}{(1+x)^2}dx (adding and subtracting 1)

\large \displaystyle \int e x ( 1 x + 1 1 ( x + 1 ) 2 ) d x e^x \Bigg(\dfrac{1}{x+1}-\dfrac{1}{(x+1)^2}\Bigg)dx

The above integral is of the form, e x ( f ( x ) + f ( x ) ) d x = e x f ( x ) \int e^x\big(f(x)+f'(x)\big)dx=e^xf(x) where f ( x ) = 1 x + 1 f(x)=\dfrac{1}{x+1}

\therefore The answer is e x x + 1 \large \dfrac{e^x}{x+1}

e x e^x is missing in the second line.

Chew-Seong Cheong - 4 years, 10 months ago

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Thanks.. Didnt see..

Sparsh Sarode - 4 years, 10 months ago

Did the same thing.

Rajdeep Ghosh - 4 years ago
Hana Wehbi
Jun 4, 2016

We need to integrate x e x ( 1 + x ) 2 d x = ? \large \displaystyle \int \dfrac{xe^x}{(1+x)^2} \, dx = \, ?

Let u = x + 1 , d u = d x + 1 u= x+1, \implies du= dx+1 ;

our integral will become x e x ( 1 + x ) 2 d x = \large \int \dfrac{xe^x}{(1+x)^2} \, dx = e 1 ( u 1 ) e u u 2 e^{-1}\large\int\frac{(u-1)e^{u}}{u^{2}} d u du ;

Now solving ( u 1 ) e u u 2 d u \large\int\frac{(u-1)e^{u}}{u^{2}}du ;

Rewrite: e u u \large\int \frac{e^{u}}{u} e u u 2 d u -\large\frac{e^{u}}{u^{2}}du ;

Apply linearity: e u u \large\int\frac{e^{u}}{u} du - e u u 2 \large\int\frac{e^{u}}{u^{2}} du;

For the integral e u u 2 \large\int\frac{e^{u}}{u^{2}} d u du , we integrate by parts: f g = f g f g \int f'g= fg- \int fg' ;

f = 1 u 2 , g \large f'= \frac{1}{u^{2}} , g = e u \large e^{u} \implies f = 1 u , g f=\frac{-1}{u} , g' = e u \large e^{u} ;

e u u \large\int\frac{e^{u}}{u} d u du f g = e u u -\large \int f'g= \large\int\frac{e^{u}}{u} d u du + f g + f g +\large fg+ \int fg' = e u d u u + ( e u d u u + e u u ) \large\int\frac{e^{u}du}{u}+(\large\int\frac{-e^{u}du}{u}+\large\frac{e^{u}}{u}) ; the signs changed because of the negative multiplication.

The integral e u d u u \large\frac{e^{u}du}{u} cancels, leaving only e u u \large\frac{e^{u}}{u} ;

Plug in solved integrals e 1 ( u 1 ) e u u 2 \large e^{-1}\int\frac{(u-1)e^{u}}{u^{2}} d u du = e u 1 u \large\frac{e^{u-1}}{u} ;

Undo substitution u = x + 1 u=x+1 yields in e x x + 1 \large\frac{e^{x}}{x+1} \implies x e x d x ( x + 1 ) 2 \large\int\frac{xe^{x}dx}{(x+1)^{2}} = e x ( x + 1 ) \large\frac{e^{x}}{(x+1)} + C +C .

Rishi K
Jun 4, 2016

Highly unorthodox method..but if its a speed-test then look at the fourth option.. By seeing it you can tell that derivative of that( quotient rule) will give you the question...waiting for someone to post the methodical answer...

totally agree. Differentiation is a better option for integration with options.. ;D

Sparsh Sarode - 5 years ago

did the same way lol!

rajdeep das - 4 years, 11 months ago

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