cos20°-cos40°-cos80°的值为?
问题描述：
cos20°-cos40°-cos80°的值为?
答
cos 40 = cos(60-20) = cos60*cos20+sin60*sin20cos 80 = cos(60+20) = cos60*cos20-sin60*sin20所以代入原式得cos20-(cos60*cos20+sin60*sin20)-(cos60*cos20-sin60*sin20)=cos20-2cos60*cos20-sin60*sin20+sin60*s...=cos20-2cos60*cos20=cos20-2*(1/2)*cos20=0; 这里看不懂厄cos60=1/2代入=cos20-2cos60*cos20=cos20-cos20=0