Ex 7.8, 22 (MCQ) - Chapter 7 Class 12 Integrals (Important Question)

Last updated at April 16, 2024 by Teachoo

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Ex 7.8, 22 Choose the correct answer ∫_0^(2/3)▒𝑑𝑥/(4 +9𝑥^2 ) equals (A) 𝜋/6 (B) 𝜋/12 (C) 𝜋/24 (D) 𝜋/4 Step 1 :- Let F(𝑥)=∫1▒𝑑𝑥/(4 + 9𝑥^2 ) Divide and multiply the integrate by 4 =∫1▒█(𝑑𝑥@4)/((4 + 9𝑥^2)/4) =1/4 ∫1▒𝑑𝑥/(1 + 9/4 𝑥^2 ) =1/4 ∫1▒𝑑𝑥/(1 + (3/2 𝑥)^2 ) Put 3/2 𝑥=𝑡 Differentiating w.r.t.𝑥 𝑑(3/2 𝑥)/𝑑𝑥=𝑑𝑡/𝑑𝑥 3/2=𝑑𝑡/𝑑𝑥 𝑑𝑥=𝑑𝑡/(3/2) 𝑑𝑥=2/3 𝑑𝑡 Hence 1/4 ∫1▒〖𝑑𝑥/(1+(3/2 𝑥)^2 )=1/4 ∫1▒〖1/(1+𝑡^2 ) 2/3〗 𝑑𝑡〗 =1/4 × 2/3 ∫1▒𝑑𝑡/(1+𝑡^2 ) =1/6 tan^(−1)⁡𝑡 Putting 𝑡=3/2 𝑥 =1/6 tan^(−1)⁡〖3/2 𝑥〗 Hence F(𝑥)=1/6 tan^(−1)⁡〖3/2 𝑥〗 Step 2 :- ∫_0^(2/3)▒〖𝑑𝑥/(4+9𝑥^2 )=𝐹(2/3)−𝐹(0) 〗 =1/6 tan^(−1)⁡〖(3/2 × 2/3)−1/6 tan^(−1)⁡(0) 〗 =1/6 tan^(−1)⁡〖(1)−1/6 tan^(−1)⁡(0) 〗 =1/6 × 𝜋/4−0 =𝜋/24 ∵ Option (C) is correct.

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Davneet Singh has done his B.Tech from Indian Institute of Technology, Kanpur. He has been teaching from the past 14 years. He provides courses for Maths, Science and Computer Science at Teachoo

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