cosα=4/5,cos(α+β)=3/5,
由于α,θ是锐角,
则sinα=3/5,sin(α+β)=4/5,
sinβ
=sin[(α+β)-α]
=sin(α+β)cosα-cos(α+β)sinα
=4/5*4/5-3/5*3/5
=7/25.