How do I evaluate this trigonometric limit involving infinite product? Tried a lot but couldn't solve it. How do I begin approaching this limit? 
$ \displaystyle\lim _{n\to\infty} \displaystyle\prod_{k=3}^{n} 1 - \tan^4\left( \frac{\pi}{2^k}\right)$
 A: There isn't that many trick to create a horrible looking and yet doable limit. 
In school, if you are asked to evaluate a series or product which involve trigonometry functions whose arguments contain terms like $\frac{(\cdots)}{2^k}$.  One thing you should do is lookup the double-angle formula for various trigonometry functions and see whether you can use them to turn your series/product into a telescoping one.
In this case, it do work.

Let $t = \tan\theta$, we have
$$1 - t^4 = (1-t^2)^2\frac{1+t^2}{1-t^2}
= \left(\frac{2t}{\frac{2t}{1-t^2}}\right)^2\frac{1+t^2}{1-t^2}$$
Recall 
$$\frac{2t}{1-t^2} = \tan(2\theta)\quad\text{ and }\quad
\frac{1-t^2}{1+t^2} = \cos(2\theta) = \frac12\frac{\sin(4\theta)}{\sin(2\theta)}
$$
We obtain
$$1 - t^4 = 8\left(\frac{\tan(\theta)}{\tan(2\theta)}\right)^2\frac{\sin(2\theta)}{\sin(4\theta)}
=  8\frac{\tan^2\theta\sin(2\theta)}{\tan^2(2\theta)\sin(4\theta)}
$$
The product is telescoping and
$$\begin{align}\prod_{k=3}^n\left(1 - \tan^4\frac{\pi}{2^k}\right)
&= 8^{n-2} \frac{\tan^2\frac{\pi}{2^n}\sin \frac{\pi}{2^{n-1}}}{\tan^2\frac{\pi}{4}\sin\frac{\pi}{2}}\\
&= \frac{1}{32} \left(2^n\tan \frac{\pi}{2^n}\right)^2\left(2^{n-1}\sin\frac{\pi}{2^{n-1}}\right)\end{align}
$$
Using
$$\lim_{N\to\infty} N\tan\frac{\pi}{N} = \lim_{N\to\infty} N\sin\frac{\pi}{N} = \pi$$
We obtain
$$\lim_{n\to\infty} \prod_{k=3}^n\left(1 - \tan^4\frac{\pi}{2^k}\right) = \frac{\pi^3}{32}$$
A: I wouldn't quickly have an idea either, but WolframAlpha gives that $\prod_{k=3}^\infty\left(1-\tan^4\left(\frac{\pi}{2^k}\right)\right)\approx0.968946$ (https://www.wolframalpha.com/input/?i=product+of+(1-tan(pi%2F2%5En)%5E4)+from+3+to+infty).
