证明cos²(A+B)—sin²(A—B)=cos2Acos2B
问题描述:
证明cos²(A+B)—sin²(A—B)=cos2Acos2B
答
左边=(cosAcosB-sinAsinB)²-(sinAcosB-cosAsinB)²=cos²Acos²B + sin²Asin²B - 2sinAsinBcosAcosB - sin²Acos²B - cos²Asin²B + 2sinAsinBcosAcosB=cos²Acos...