α,β均為銳角,
cosα=4/5 sinα=3/5
cos（α+β）=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
cosβ=√(1-sin^2β)=24/25
sin2β=2sinβcos2β=2*7/25*24/25=336/625