Question

cos20°-cos40°-cos80°的值为?

Answer 1

cos 40 = cos(60-20) = cos60*cos20+sin60*sin20

cos 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*sin20

=cos20-2cos60*cos20

=cos20-2*(1/2)*cos20

=0;