2cos（α+π/4）=2[cosαcos(π/4)-sinαsin（π/4）]=2[(√2/2)cosα-(√2/2)sinα]=√2(cosα-sinα)因为α属于（0,π/2）sinα=3/5、所以cosα=4/5所以√2(cosα-sinα)=]=√2/5 老兄、α属于（0,π/2）吧
