利用二倍角公式即可
cos36°×cos72°
=sin36°*cos36°×cos72°/(sin36°)
=(1/2)sin72°×cos72°/(sin36°)
=(1/4)sin144°/sin36°
=(1/4)sin(180°-36°)/sin36°
=(1/4)sin36°/sin36°
=1/4