cosa=4/5,sina=3/5cos(a+b)=cosacosb-sinasinb3/5=4/5cosb-3/5sinb3=4cosb-3sinb(3+3sinb)^2=16cos^2b=16(1-sin^2b)9+18sinb+9sin^b=16-16sin^2b25sin^2b+18sinb-7=0(25sinb-7)(sinb+1)=0sinb=7/25 sinb=-1(舍去）...