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Question

If cot1(n210n+21.6π)>π6,nϵN terms, then n can be

A
3
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B
2
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C
4
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D
8
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Solution

The correct options are
A 4
B 3
Taking cot on both the sides, we get
n210n+21.6<πcot(π6) ...( inequality sign gets reversed, since cot(x) is a decreasing function).
n210n+21.6<π(3)
n210n+21.6<5.44
n210n+16.19<0
Consider
g(n)=n210n+16.19
g(2)=420+16.19
=0.19 ...(i)
g(3)=930+16.19
=4.81<0 ...(ii)
g(4)=1640+16.19
=7.81<0 ...(iii)
g(8)=6480+16.19
=0.19 ...(iv)
Hence only 3, and 4 satisfies the above inequality,

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