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Definite Integration question

2018 · Shift 2 · Q25
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  5. /2018 · Shift 2 · Q25

Definite Integration question

2018 · Shift 2 · Q25

JEE AdvancedMathematicsDefinite IntegrationNumerical+3 / −1
The value of the integral ∫01/21+3((x+1)2(1−x)6)1/4dx\int_0^{1/2} {{{1 + \sqrt 3 } \over {{{({{(x + 1)}^2}{{(1 - x)}^6})}^{1/4}}}}dx}∫01/2​((x+1)2(1−x)6)1/41+3​​dx is ........
Numerical answer
View written solutionFree

Correct answer: 2

  1. Rewrite the integrand

Given I=∫01/21+3((x+1)2(1−x)6)1/4 dxI=\int_0^{1/2} \frac{1+\sqrt3}{\left((x+1)^2(1-x)^6\right)^{1/4}}\,dxI=∫01/2​((x+1)2(1−x)6)1/41+3​​dx

First simplify the denominator: ((x+1)2(1−x)6)1/4=(x+1)1/2(1−x)3/2\left((x+1)^2(1-x)^6\right)^{1/4}=(x+1)^{1/2}(1-x)^{3/2}((x+1)2(1−x)6)1/4=(x+1)1/2(1−x)3/2 for 0≤x≤120\le x\le \tfrac120≤x≤21​.

So, I=(1+3)∫01/2dxx+1(1−x)3/2I=(1+\sqrt3)\int_0^{1/2}\frac{dx}{\sqrt{x+1}(1-x)^{3/2}}I=(1+3​)∫01/2​x+1​(1−x)3/2dx​


  1. Look for a useful substitution

Let t=x+11−xt=\sqrt{\frac{x+1}{1-x}}t=1−xx+1​​ Then t2=x+11−xt^2=\frac{x+1}{1-x}t2=1−xx+1​

Differentiate implicitly: 2t dtdx=(1−x)−(−(x+1))(1−x)2=2(1−x)22t\,\frac{dt}{dx}=\frac{(1-x)-(-(x+1))}{(1-x)^2}=\frac{2}{(1-x)^2}2tdxdt​=(1−x)2(1−x)−(−(x+1))​=(1−x)22​ Thus, dtdx=1t(1−x)2\frac{dt}{dx}=\frac{1}{t(1-x)^2}dxdt​=t(1−x)21​

Now since t=x+11−x,t=\frac{\sqrt{x+1}}{\sqrt{1-x}},t=1−x​x+1​​, we get t(1−x)2=x+1(1−x)3/2t(1-x)^2=\sqrt{x+1}(1-x)^{3/2}t(1−x)2=x+1​(1−x)3/2 Hence dt=dxx+1(1−x)3/2dt=\frac{dx}{\sqrt{x+1}(1-x)^{3/2}}dt=x+1​(1−x)3/2dx​

So the integral becomes simply I=(1+3)∫dtI=(1+\sqrt3)\int dtI=(1+3​)∫dt


  1. Change the limits

When x=0x=0x=0, t=11=1t=\sqrt{\frac{1}{1}}=1t=11​​=1

When x=12x=\frac12x=21​, t=3212=3t=\sqrt{\frac{\frac32}{\frac12}}=\sqrt3t=21​23​​​=3​

Therefore, I=(1+3)∫13dt=(1+3)(3−1)I=(1+\sqrt3)\int_1^{\sqrt3} dt=(1+\sqrt3)(\sqrt3-1)I=(1+3​)∫13​​dt=(1+3​)(3​−1)


  1. Simplify

(1+3)(3−1)=3−1=2(1+\sqrt3)(\sqrt3-1)=3-1=2(1+3​)(3​−1)=3−1=2

So, 2\boxed{2}2​


  1. Comparison with stored correct answer

Derived answer = 222

Stored correct answer = 222

They match.

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