由β∈(π/2,π),cosβ=－5/13,可知sinβ=根号(1-cos²β）=12/13由α∈(0,π/2),可知α+β∈（π/2,3π/2）,则cos(α+β)=[1-sin²(α+β)]=-56/65∴sinα=sin[(α+β)-β]=sin(α+β)cosβ-cos(α+β)sin...