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JEE Main 202315 AprilMorning Shift
Exam
JEE Main
Year
2023
Date
15 April
Shift
Morning Shift
Subject
Mathematics
Chapter
Indefinite Integration

JEE Main 2023 · 15 April · Morning Shift · Mathematics · Indefinite Integration

Let f(x) = ∫ dx (3 + 4 x ^ 2 ) 4 - 3 x ^ 2 , |x| 0 , then α ^ 2 + β...

Let f(x)=∫dx(3+4x2)4−3x2,∣x∣<23f\left(x\right) = ∫ \dfrac{dx}{\left(3 + 4{x}^{2}\right)\sqrt{4 - 3{x}^{2}}}, \left|x\right| < \dfrac{2}{\sqrt{3}}. If f(0)=0f\left(0\right) = 0 and f(1)=1αβtan⁡−1(αβ),α,β>0f\left(1\right) = \dfrac{1}{\alpha\beta}\tan^{- 1}\left(\dfrac{\alpha}{\beta}\right), \alpha, \beta > 0, then α2+β2{\alpha}^{2} + {\beta}^{2} is equal to _______.

Correct answer

0

Step-by-step solution

Given,

f(x)=∫dx(3+4x2)4−3x2f\left(x\right) = ∫ \dfrac{dx}{\left(3 + 4{x}^{2}\right)\sqrt{4 - 3{x}^{2}}}

Put x=1t,dx=−1t2dtx = \dfrac{1}{t}, dx = - \dfrac{1}{{t}^{2}}dt

So, f(x)=∫−dtt2(3+4t2)4−3t2f\left(x\right) = ∫ \dfrac{- dt}{{t}^{2}\left(3 + \dfrac{4}{{t}^{2}}\right)\sqrt{4 - \dfrac{3}{{t}^{2}}}}

⇒f(x)=∫−tdt(3t2+4)4t2−3⇒ f\left(x\right) = ∫ \dfrac{- tdt}{\left(3{t}^{2} + 4\right)\sqrt{4{t}^{2} - 3}}

Now let, 4t2−3=λ2⇒8tdt=2\lambdadλ4{t}^{2} - 3 = {\lambda}^{2} ⇒ 8tdt = 2\lambdad\lambda

⇒f(x)=−∫\lambdadλ4⋅(3(λ2+34)+4)⋅λ⇒ f\left(x\right) = - ∫ \dfrac{\lambdad\lambda}{4 \cdot \left(3\left(\dfrac{{\lambda}^{2} + 3}{4}\right) + 4\right) \cdot \lambda}

⇒f(x)=−∫dλ3λ2+9+16⇒ f\left(x\right) = - ∫ \dfrac{d\lambda}{3{\lambda}^{2} + 9 + 16}

⇒f(x)=−∫dλ3λ2+25⇒ f\left(x\right) = - ∫ \dfrac{d\lambda}{3{\lambda}^{2} + 25}

⇒f(x)=−13∫dλλ2+253⇒ f\left(x\right) = - \dfrac{1}{3} ∫ \dfrac{d\lambda}{{\lambda}^{2} + \dfrac{25}{3}}

⇒f(x)=−13×35tan⁡−1(3λ5)+c⇒ f\left(x\right) = - \dfrac{1}{3} \times \dfrac{\sqrt{3}}{5}\tan^{- 1}\left(\dfrac{\sqrt{3}\lambda}{5}\right) + c

⇒f(x)=−315tan⁡−1(3(4−3x2)5x)+c⇒ f\left(x\right) = - \dfrac{\sqrt{3}}{15}\tan^{- 1}\left(\dfrac{\sqrt{3}\sqrt{\left(4 - 3{x}^{2}\right)}}{5x}\right) + c

Now using, f(0)=0⇒c=+3π30f\left(0\right) = 0 ⇒ c = + \dfrac{\sqrt{3}\pi}{30}

Hence, f(1)=−315tan⁡−1(35)+315×π2f\left(1\right) = \dfrac{- \sqrt{3}}{15}\tan^{- 1}\left(\dfrac{\sqrt{3}}{5}\right) + \dfrac{\sqrt{3}}{15} \times \dfrac{\pi}{2}

⇒f(1)=−315(tan⁡−1(35)−π2)⇒ f\left(1\right) = \dfrac{- \sqrt{3}}{15}\left(\tan^{- 1}\left(\dfrac{\sqrt{3}}{5}\right) - \dfrac{\pi}{2}\right)

⇒f(1)=315(π2−tan⁡−1(35))⇒ f\left(1\right) = \dfrac{\sqrt{3}}{15}\left(\dfrac{\pi}{2} - \tan^{- 1}\left(\dfrac{\sqrt{3}}{5}\right)\right)

⇒f(1)=315cot⁡−1(35)⇒ f\left(1\right) = \dfrac{\sqrt{3}}{15}\cot^{- 1}\left(\dfrac{\sqrt{3}}{5}\right)

⇒f(1)=315tan⁡−1(53)⇒ f\left(1\right) = \dfrac{\sqrt{3}}{15}\tan^{- 1}\left(\dfrac{5}{\sqrt{3}}\right)

⇒f(1)=153tan⁡−1(53)⇒ f\left(1\right) = \dfrac{1}{5\sqrt{3}}\tan^{- 1}\left(\dfrac{5}{\sqrt{3}}\right)

Now comparing with given value of f(1)f\left(1\right) we get, α=5 and β=3\alpha = 5 \text{ and } \beta = \sqrt{3}

⇒α2+β2=28⇒ {\alpha}^{2} + {\beta}^{2} = 28

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