若锐角a,b满足cosa=4/5,cos(a+b)=3/5,则sinb=?
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若锐角a,b满足cosa=4/5,cos(a+b)=3/5,则sinb=?
若锐角a,b满足cosa=4/5,cos(a+b)=3/5,则sinb=?
若锐角a,b满足cosa=4/5,cos(a+b)=3/5,则sinb=?
cosa=4/5,sina=3/5
cos(a+b)=cosacosb-sinasinb
3/5=4/5cosb-3/5sinb
3=4cosb-3sinb
(3+3sinb)^2=16cos^2b=16(1-sin^2b)
9+18sinb+9sin^b=16-16sin^2b
25sin^2b+18sinb-7=0
(25sinb-7)(sinb+1)=0
sinb=7/25 sinb=-1(舍去)
答案不错
一楼正解