求证cos(a+b)*cos(a-b)=cos^2 a-sin^2 b 求详解
问题描述:
求证cos(a+b)*cos(a-b)=cos^2 a-sin^2 b 求详解
答
cos(a+b)=cosacosb-sinasinbcos(a-b)=cosacosb+sinasinb所以cos(a+b)*cos(a-b)=cos^2acos^2b-sin^2asin^2b=cos^2a(1-sin^2b)-sin^2asin^2b =cos^2a-cos^...