cos(α-3π/2)=cosαcos3π/2+sinαsin3π/2=-sinα=1/5
sinα=-1/5 α在二、三象限
(cosα)^2=1-(-1/5)^2=24/25
α在二、三象限
cosα=-（2√6）/5
-cosα=（2√6）/5