sina+cosa=1/5,sina=1/5-cosa于是sin^2a+cos^2a=(1/5-cosa)^2+(cosa)^2=1解得cosa=-3/5或4/5但当cosa=4/5,时,sina=-3/5,因为a在（0°,180°）内,sina>0,因此此解舍去于是cosa=-3/5,sina=4/5tana=sina/cosa=-4/3...