find the limit of the sequence :
$\displaystyle
\lim_{x\to \infty}(cos\frac{x}{2}cos\frac{x}{4}...cos\frac{x} {2^n})$
I believe what you ask about is know as Vičte's infinite product
see List of trigonometric identities - Wikipedia, the free encyclopedia
a little more than half way down.