已知α,β∈（3π/4,π),cos（α+β)=4/5,sin(β-π/4)=12/13,求cos(α+π/4)

cos（α+β)=cos（(α+π/4)+(β-π/4))=cos(α+π/4)cos（β-π/4)-SIN(α+π/4)SIN（β-π/4)=4/5sin（α+β)=sin（(α+π/4)+(β-π/4))=sin(α+π/4)cos（β-π/4)+cos(α+π/4)SIN（β-π/4)=-3/5(IV象限)COS(...