cos(a-六分之π)=三分之根号三.那么sin(2a+六分之π)多少
问题描述：
cos(a-六分之π)=三分之根号三.那么sin(2a+六分之π)多少
答
cos(a-π/6)=根号3/3
cos[2(a-π/6)]=cos(2a-π/3)=2[cos(a-π/6)]^2-1=-1/3
sin(2a+π/6)=sin[π/2+(2a-π/3)]=cos(2a-π/3)=-1/3