已知sin(30°+α)=1/3,求cos(120°-2α)的值
问题描述：
已知sin(30°+α)=1/3,求cos(120°-2α)的值
答
cos(120°-2α)
=cos(180°-60°-2α)
=－cos(60°+2α)
=－cos2(30°+α)
=－(1-2(sin(30°+α))^2)
=－(1-2(1/3)^2)
=－7/9