∵α,β都是锐角　　∴sinα>0,sin(α+β)>0从而 (sinα)^2=1-(cosα)^2=1-(5/13)^2=(12/13)^2sin(α+β)^2=1-cos(α+β)^2=1-(-1/3)^2=8/9得 sinα=12/13sin(α+β)=2√2/3由 sin(α+β)=sinα·cosβ+cosα·sinβ...