(sina+cosa)^2=1/25
2sinacosa=1/25-sin^2+cos^2=-24/25
sinacosa=-12/25
(sina-cosa)^2=sin^2+cos^2-2sinacosa
　　                     =49/25
因为a∈（0,π）所以sina<cosa
sina-cosa=-7/5
tan（a-π/2）=tana
sina+cosa=1/5
               
sina-cosa=-7/5
合并sina=-3/5,cosa=4/5
tana=sina/cosa=-3/4