sin3α·cosα=2sinα·cosαsin2α·cosα+cos2α·sinα=2sinα2sinα·(cosα)^2+(1-2(sinα)^2)·sinα=2sinα(1-2(sinα)^2)·sinα=2sinα·(1-(cosα)^2)(1-2(sinα)^2)·sinα=2(sinα)^31-2(sinα)^2=2(sinα)^2(sinα)^2=1/4sinα=正负1/2α=kπ加减π/6（k为整数）