∵0≤α≤π ∴sina>0 ∵sina+cosa=-1/5 ∴cosa0sina+cosa=-1/5 (1)sin²a+cos²a=1 (2)(1)²-(2)得 2sinacosa=-24/25 (3)(2)-(3)得 (sina-cosa)²=49/25∵sina-cosa>0 （上已证）∴sina-cosa=7/5 (...