cos兀/9×cos2兀/9×cos(-23兀/9)

问题描述:

cos兀/9×cos2兀/9×cos(-23兀/9)

原式乘以sinπ/9=1/2sin2π/9 ×cos2兀/9×cos(-23兀/9)=1/4sin4π/9×(-cos4π/9)=-1/8sin8π/9=-1/8sinπ/9
所以原式=(-1/8sinπ/9)/sinπ/9=-1/8