cos(α-3/2π)=1/5,则：
cos(α-3/2π)=cos(α-3/2π+2π)=cos(α+1/2π)=sinα=1/5,
又(sinα)^2+(cosα)^2=1,
所以 (cosα)^2=1-(1/5)^2=24/25,
所以cosα=2√6/5,或cosα=-2√6/5.