0所以cosa=4/5 cos(a b)=cosacosb-sinasinb=-4/5 cosb4/5-3/5sinb=-4/5 4cosb-3sinb=-4 cosb=(3sinb-4)/4 cos^b=1-sin^2b=(3sinb-4)^2/16 16-16sin^2b=9sin^2b 16-24sinb 24sinb=25sin^2b sinb不等于0 哪里不清请追问，满意请采纳，谢谢~~ 所以sinb=24/25
