已知sin(x-15°)=1/3,求cos(30°一2x)
问题描述：
已知sin(x-15°)=1/3,求cos(30°一2x)
答
cos(30°-2x)=cos(2x-30°)=cos[2(x-15°)]=1-2sin^2(x-15°)=1-2*(1/3)^2=7/9
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答
cos(30-2x)=cos(2x-30)=1-2sin(x-15)^2=7/9