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Question

The value ofsin 5αsin 3αcos 5α+2 cos 4α+cos 3α is


A

cot α2

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B

cot α

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C

tan α2

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D

none of these

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Solution

The correct option is C

tan α2


sin 5αsin 3αcos 5α+2 cos 4α+cos 3α=sin 5αsin 3αcos 5α+cos 3α+2 cos 4α=2 sin α cos 4α2 cos 4α cos α+2 cos 4α=2 sin α cos 4α2 cos 4 α(cos α+1)=sin αcos α+1

=2 sin α2.cosα2cos2 α2sin2 α2+sin2 α2+cos2 α2=2 sin α2 cos α22 cos2 α2=sinα2cosα2=tan α2


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