sinα=-3/5,α∈（-π/2,0）cosα=根号(1-sin^2)=根号[1-(-3/5)^]=4/5tanα=sinα/cosα=-3/5 / (4/5) = -3/4tan(π/4-α)=(tanπ/4-tanα)/(1+tanπ/4tanα)=(1-tanα)/(1+tanα)=(1+3/4)/(1-3/4)=7