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Question

sin2α+sin2βcos2αcos2β=cot(βα)

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Solution

sin2α+sin2βcos2αcos2β=cot(βα)
L.H.S=sin2α+sin2βcos2αcos2β
=2sin(2α+2β2)cos(2α2β2)2sin(2α+2β2)sin(2β2α2)
[using sinC+sinD=2sin(C+D2)cos(CD2)
and cosCcosD=2sin(C+D2)sin(DC2)]
L.H.S becomes =cos(2α2β2)sin(2β2α2)
=cos(αβ)sin(βα)=cos(βα)sin(βα)
[as cos(θ)=cosθ]
cot(βα)=R.H.S.

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