sin(π/4+a)sin(π/4-a)=1/6求cos2a

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sin(π/4+a)sin(π/4-a)=1/6求cos2asin(π/4+a)sin(π/4-a)=1/6求cos2asin(π/4+a)sin(π/4-a)=1/6求cos2a(sinπ/4cos

sin(π/4+a)sin(π/4-a)=1/6求cos2a
sin(π/4+a)sin(π/4-a)=1/6求cos2a

sin(π/4+a)sin(π/4-a)=1/6求cos2a
(sinπ/4cosa+cosπ/4sina)(sinπ/4cosa-cosπ/4sina)=1/6
[√2/2*(cosa+sina)][√2/2*(cosa-sina)]=1/6
1/2*(cos²a-sin²a)=1/6
cos2a=cos²a-sin²a=1/3

sin(π/4+a)sin(π/4-a)=1/6
2sin(π/4+a)sin(π/4-a)=1/3
sin(π/2+2a)=1/3
cos2a=sin(π/2+2a)=1/3