α∈(π/2,π),α-π/4∈(π/4,3π/4)cos(∏/4-α)=cos(α-π/4)=3/5>0所以α-π/4∈(π/4,π/2)sin(α-π/4)=4/5cosα = cos(α-π/4+π/4)=cos(α-π/4)cos(π/4)-sin(α-π/4)sin(π/4)=3/5*√2/2-4/5*√2/2=-√2/10
