L.H.S.=cos2x+cos2(x+π3)+cos2(x−π3)=1+cos 2x2+1+cos(2x+2π3)2+1+cos(2x−2π3)2=12(3+cos 2x+cos(2x+2π3)+cos(2x−2π3))=12⎛⎜
⎜
⎜⎝3+cos2x+2cos⎛⎜
⎜
⎜⎝2x+2π3+2x−2π32⎞⎟
⎟
⎟⎠cos⎛⎜
⎜
⎜⎝2x+2π3−2x+2π32⎞⎟
⎟
⎟⎠⎞⎟
⎟
⎟⎠=12(3+cos2x+2cos(2x)cos(2π3))=12(3+cos2x−2cos(2x)(12))=12(3+cos2x−cos(2x))=32=R.H.S
Hence proved.