1. cos2α = 2cosα× cosα -1=2×4/5 -1= 3/52. π/2<斗盯β<π, cosβ=(-)5/13 ,所空乱和以sinβ= 12/13 0<α<π/2, cosα=4/5, 所以 sina=3/陪塌5 sin(α+β)=sinαcosβ+cosαsinβ) =3/5 * (-)5/13 + 4/5* 12/13= 33/65