sinα=2/3 α∈(π/2,π)cosβ=-3/5,β∈(π,3π/2)
cosα=(√5)/3,sinβ=-4/5
cos(α-β)=cosαcosβ+sinαcosβ
sin(α+β)=sinαcosβ+sinβcosα
∵α∈(π/2,π),β∈(π,3π/2)
∴cosα<0,sinβ<0
cosα = -√(1-(2/3)²) = -√5/9
sinβ = -√(1-(-3/5)²) = -4/5
cos(α-β)=cosαcosβ+sinαcosβ
sin(α+β)=sinαcosβ+sinβcosα
cosα=-5½/3 sinβ=-4/5;
cos(α-β)=3*5½/15-8/15;
sin(α+β)=-6/15+4*5½/15