If π∫0dxa−cosx=π√a2−1, then the value of π∫0dx(√10−cosx)3 is equal to (where |a|>1)
A
π81
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B
7π162
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C
7π81
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D
π162
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Solution
The correct option is B7π162 π∫0dx(a−cosx)=π√a2−1
Differentiating both sides with respect to a, we get −π∫0dx(a−cosx)2=−πa(a2−1)32
Again differentiating with respect to a, we get 2π∫0dx(a−cosx)3=π(1+2a2)(a2−1)52
Putting a=√10, we get π∫0dx(√10−cosx)3=7π162