cos2α =1 -2(sinα)^2
= 1 - 2 *1/4
=1/2
由“ cos2α=1/2”推出sinα= 1/2或-1/2
cos2α =1 -2(sinα)^2 =1/2
(sinα)^2=1/4
sinα=1/2 或-1/2