A trick problem !!

In the expression $\large \cos \frac{\pi}{28} \cos \frac{2 \pi}{28} .... \cos \frac{28 \pi}{28}=a$ , you can see that all the values are in multiplication form.

Also, note that there will be an element $\large \cos \frac{14 \pi}{28}$ in the series on LHS of the equation which is actually equal to $\large \cos \frac{\pi}{2}$ whose value is $0$ . Now, $0$ multiplied by the rest of the expression on LHS gives $0$ since $\boxed{0\times \text{ (Any value) }=0}$ .

This results in the equation becoming $\boxed{a=0}$