sinα=12/13
(sinα)^2+(cosα)^2=1
且α∈(0,π/2)
所以cosα＞0
cosα=5/13
cosβ=-3/5
(sinα)^2+(cosα)^2=1
且β∈(π,3π/2)
所以sinβ＜0
sinβ=-4/5
sin（α+β）=sinαcosβ+sinβcosα
=-56/65
题是纯手打啊
不懂可以继续追问啊