Question

cos540*cos600*cos780

Answer 1

$cos540\cdot cos600\cdot cos780=\\\\=cos(360+180)\cdot cos(720-120)\cdot cos(720+60)=\\\\=cos180\cdot cos(-120)\cdot cos60=-1\cdot cos120\cdot \frac{1}{2}=-\frac{1}{2}\cdot cos(180-60)=\\\\=-\frac{1}{2}\cdot (-cos\, 60)=-\frac{1}{2}\cdot (-\frac{1}{2})=\frac{1}{4}$