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|DATE :1 1e vd

Answer»

we have identity cos (A-B)=cosA cosB +sinA sinBsosin3A sinA+cos3A cosA=cos (3A-A)=cos2Atherefore, sin3A sinA+cos3A cosA+2sin^2 A=cos2A+2sin^2 A=1-2sin^2 A+2sin^2 A....... (using cos2A=1-2sin^2 A)=1



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