已知sin(π/6+α)=3/5,求cos(π/3+2α)的值.要细节

题目

已知sin(π/6+α)=3/5,求cos(π/3+2α)的值.要细节

答案

cos(π/3+2α)=cos2(π/6+α)=1-2sin^2(π/6+α)=1-2*(3/5)^2=7/25