若sin(π/6-a)=1/3 则cos(2/3π+2a)的值为
问题描述：
若sin(π/6-a)=1/3 则cos(2/3π+2a)的值为
答
cos(2π/3+2a)
=cos[2(π/3+a)]
=2cos²(π/3+a)-1
=2sin²[π/2-(π/3+a)]-1
=2sin²(π/6-a)-1
=-7/9