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Question

Prove that cos2x+cos2(x+π3)+cos2(xπ3)=32

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Solution

L.H.S.=cos2x+cos2(x+π3)+cos2(xπ3)=1+cos 2x2+1+cos(2x+2π3)2+1+cos(2x2π3)2=12(3+cos 2x+cos(2x+2π3)+cos(2x2π3))=12⎜ ⎜ ⎜3+cos2x+2cos⎜ ⎜ ⎜2x+2π3+2x2π32⎟ ⎟ ⎟cos⎜ ⎜ ⎜2x+2π32x+2π32⎟ ⎟ ⎟⎟ ⎟ ⎟=12(3+cos2x+2cos(2x)cos(2π3))=12(3+cos2x2cos(2x)(12))=12(3+cos2xcos(2x))=32=R.H.S

Hence proved.

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