Cos(a+b)*cos(a-b)=1/5 求cos ^2-sin^2
问题描述：
Cos(a+b)*cos(a-b)=1/5 求cos ^2-sin^2
答
原题是这样子吧：cos(a+b)cos(a-b)=1/5,则(cosa)^2-(sinb)^2=?cos(a+b)cos(a-b)=(cosacosb-sinasinb)(cosacosb+sinasinb)=(cosacosb)^2-(sinasinb)^2=cosa^2[1-(sinab)^2]-sinb^2[1-(cosa)^2]=(cosa)^2-(sinb)^2=1/5...