sinα+cosα=1/5(1)sin^2α+cos^2α=1(2),由(1)得cosα=1/5-sinα, 代入(2)得,sin^2α+(1/5-sinα)^2=1, 整理得25sin^2α-5sinα-12=0,所以sinα=4/5,代入(1)得cosα=-3/5tan(3π-α)=tan(-α)=-tanα=-sinα/co...