cos(兀/4+α)=-3/5.cosα-sinα=-3√2/5.平方得,sinαcosα=7/50.配方得,cosα+sinα=4√2/5.∴cosα=√2/10.sinα=7√2/10.同理,从sin(3兀/4+β)=5/13,展开,平方,配方得.cosβ=17√2/26.sinβ=√2/26.sin(β+α)＝7...