$\cos 6^0 \cos {42}^0 \cos {66}^0 \cos {78}^0 =$

$\begin{array}{rcl} & & \cos 6^0 \cos {42}^0 \cos {66}^0 \cos {78}^0\\ & =& \frac{\cos 6^0 \cos {66}^0 \cos {54}^0}{\cos {54}^0}·\frac{\cos {18}^0 \cos {42}^0 \cos {78}^0}{\cos {18}^0}\\ & =& \frac{{\displaystyle \frac{1}{4}}\cos {18}^0}{\cos {54}^0}·\frac{{\displaystyle \frac{1}{4}}\cos {54}^0}{\cos {18}^0}\\ & =& \frac{1}{16}\end{array}$