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Question

If X=sin(θ+7π12)+sin(θπ12)+sin(θ+3π12),Y=cos(θ7π12)+cos(θπ12)+cos(θ+3π12)provethatxyyx=2tan2θ.

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Solution

x=sin(θ+7π12)+sin(θπ12)+sin(θ+3π12)

2sin(2θ+π22)cos(2π32)+sin(θ+3π12)

=2sin(θ+π4)cos(π3)+sin(θ+π4)

=2sin(θ+π4).12+sin(θ+π4)

=2sin(θ+π4)

y=cos(θ+7π12)+cos(θπ12)+cos(θ+3π12)

=2cos(2θ+π/22)cos(2π/33)+cos(θ+π4)
=2cos(θ+π/4).12+cos(θ+π/4)

=2cos(θ+π/4)

Now xyyx=2sin(θ+π/4)2cos(θ+π/4)2cos(θ+π/4)2sin(θ+π/4)

=sin2(θ+π/4)cos2(θ+π/4)sin(θ+π/4)cos(θ+π/4)

=2cos(θ+π/2)2sin(θ+π/4)cos(θ+π/4)

=2sin2θsin(2θ+π/2)=2sin2θcos2θ

=2tan2θ


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