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Q.If ∫2xdttt2−1=π12\displaystyle\int_{\sqrt2}^{x} \dfrac{dt}{t\sqrt{t^2-1}} = \dfrac{\pi}{12}, then the value of x is

(a) 2
(b) 1
(c) 0
(d) -1
Manipur CohsemCOHSEM Manipur Higher Secondary Board 2025MCQ· 1mImportance★★★★★
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The antiderivative of 1tt2−1\dfrac{1}{t\sqrt{t^2-1}} is sec⁡−1t\sec^{-1}t; apply the limits and solve for xx.

We know the standard result:

∫dttt2−1=sec⁡−1∣t∣+C\int\dfrac{dt}{t\sqrt{t^2-1}} = \sec^{-1}|t| + C

So

∫2xdttt2−1=sec⁡−1x−sec⁡−12\int_{\sqrt2}^{x}\dfrac{dt}{t\sqrt{t^2-1}} = \sec^{-1}x - \sec^{-1}\sqrt2

Since sec⁡π4=2\sec\dfrac{\pi}{4} = \sqrt2, we have sec⁡−12=π4\sec^{-1}\sqrt2 = \dfrac{\pi}{4}.

Given this equals π12\dfrac{\pi}{12}:

sec⁡−1x−π4=π12\sec^{-1}x - \dfrac{\pi}{4} = \dfrac{\pi}{12} …

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