cos(a+b)=1/3 cos(a-b)=0.2求tanatanb

网友回答

cosacosb-sinasinb=1/3
cosacosb+sinasinb=1/5
相加除2cosacosb=4/15
相减除2sinasinb=-1/15
tanatanb=(sina/cosa)*(sinb/cosb)
=sinasinb/cosacosb
=-1/4======以下答案可供参考======
供参考答案1:
cos(a+b)=1/3
cosacosb-sinasinb=1/3
cos(a-b)=1/5
cosacosb+sinasinb=1/5
两式相加得2cosacosb=8/15
cosacosb=4/15
所以sinasinb=cosacosb-1/3=4/15-1/3=-1/15
tanatanb=(sinasinb)/(cosacosb)=(-1/15)/(4/15)=-1/4