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Question

Prove cos2θ+cos2(θ+120)+cos2(θ120)=32

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Solution

cos2θ+cos2(θ+120)+cos2(θ120)

=1+cos2θ2+1+cos(2θ+240)2+1+cos(2θ240)2

=32+12[cos2θ+cos(2θ+240)+cos(2θ240)]

=32+12[cos2θ+2cos2θcos240]

=32+12[cos2θ+2cos2θcos(180+60)]

=32+12[cos2θ2cos2θcos60]

=32+12[cos2θcos2θ]

=32+0

=32

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