已知√3sinα+cosα=2/3,则cos(2α+π/3)等于
问题描述：
已知√3sinα+cosα=2/3,则cos(2α+π/3)等于
答
√3sinα+cosα=2/3,
2*[(√3/2)sinα+(1/2)cosα]=2*(1/3),
2*sin(π/6+α)=2*(1/3)
sin(π/6+α)=1/3
cos(π/3+2α)=1-2[sin(π/6+α)]^2=1-2/9=7/9