cosα=-4/5,α属于(π/2,π),∴ sinα>0∴ sinα=√(1-cos²α)=√(1-16/25)=3/5∴ tanα=sinα/cosα=(3/5)/(-4/5)=-3/4∴ tan(π/4-α)=[tan(π/4)-tanα]/[1+tan(π/4)tanα]=(1-tanα)/(1+tanα)=(1+3/4)/(1...