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=?
数学人气：184 ℃时间：2019-10-23 13:26:32
优质解答
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）...
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