sinα=4/5, => cosα=3/5tanβ=1/7, => sinβ= 1/√8 , cosβ= 7/√8sin(α-β) = sinαcosβ-cosαsinβ =(4/5)(7/√8)-(3/5)(1/√8) = 5/√8 = 5√2/8 α-β = arcsin(5√2/8)