Question

cos(a+b)=1/3 cos(a-b)=0.2求tanatanb？

Answer 1

cosaco *** -sinasinb=1/3
cosaco *** +sinasinb=1/5
相加除2
cosaco *** =4/15
相减除2
sinasinb=-1/15
tanatanb=(sina/cosa)*(sinb/co *** )
=sinasinb/cosaco ***
=-1/4,2,cos(a+b)=1/3
cosaco *** -sinasinb=1/3
cos(a-b)=1/5
cosaco *** +sinasinb=1/5
两式相加得
2cosaco *** =8/15
cosaco *** =4/15
所以sinasinb=cosaco *** -1/3=4/15-1/3=-1/15
tanatanb=(sinasinb)/(cosaco *** )=(-1/15)/(4/15)=-1/4,1,