sin(α+β)=5/13 sinα=4/5 α是锐角 β是钝角 求cosβ首先：cosα= 3/5sin(α+β)= sinαcosβ + cosαsinβ = 4/5cosβ + 3/5sinβ = 5/13设t = cosβ 有 4t/5 + 3根号(1-t^)/5 = 5/13有 3根号(1-t^)/5 = 5/13 - 4...