2cos(180-20)*cos(180-40)*cos(90+10)= -2cos20*(-cos40)*(-sin10)=-2sin10*((cos(20+40)+cos(40-20))/2)= 

=-sin10*(cos60+cos20)=-sin10*(1/2+cos20)= -1/2sin10-(sin(10+20)+sin(10-20))/2=-1/2sin10-1/2sin30-1/2(-sin10)= =-1/2sin30=-1/4
Ответ: -1/4