sinα=1/3 则sin3α=
问题描述：
sinα=1/3 则sin3α=
答
sin3a -sina = sin(a+2a) + sin(-a) = sin(a+2a) + sin(a-2a)= 2sinacos2acos2a = 1-2sin²a = 1- 2*1/9 = 7/9sin3a -sina = 2 * 1/3 * 7/9 = 14/27sina = 14/27 + 1/3 = 14/27 + 9/27 = 23/27