请教2cos18°×cos18°(sin18°-1)+cos36°sin18°的值,

题目

请教2cos18°×cos18°(sin18°-1)+cos36°sin18°的值,

答案

原式=2cos18sin18cos18-2cos18cos18+cos36sin18=sin36cos18+cos36sin18-1+(1-2cos18cos18)=sin54-1-cos36=-1