sina+cosa=1/5.(1)
sina^2+cosa^2=1.(2)
(1)^2-(2)得：2sinacosa=-24/25
0≤α0,cosa0
sina-cosa=√(sin^2a+cos^2a-2sinacosa)=√(1+24/25)=7/5.(3)
由(1)(3)得sina=4/5,cosa=-3/5
tans=sina/cosa=-4/3