cos(a+B)×cos(a-B)=1/3,求cos^2(a)-sin^2(B)的值
问题描述：
cos(a+B)×cos(a-B)=1/3,求cos^2(a)-sin^2(B)的值
答
cos^2 a-sin^2 b=(1+cos2a)/2-(1-cos2b)/2
=(cos2a+cos2b)/2=cos(a+b)cos(a-b)=1/3