# The value of the following product is?

Evaluate the following product: $$\newcommand{\T}{\frac{\sin\frac{\theta}{#1}}{\tan^2\frac{\theta}{#1}\tan\frac{2\theta}{#1} + \tan\frac{\theta}{#1}}} \\ P(\theta) = \T{2} \times \T{2^2} \times \T{2^3} \times .... \infty$$ For $$\theta = \frac \pi 4$$

Simplified, $$P(\theta)$$ is $$P(\theta) = \lim_{n \to \infty}\prod_{r=1}^n T(\theta,r)= \lim_{n \to \infty}\prod_{r=1}^n\T{2^r}$$ The denominator can be simplified as follows: $$D = \tan\frac{\theta}{2^r}\left( \tan\frac{\theta}{2^r}\tan\frac{\theta}{2^{r-1}} + 1\right) \\ = \tan\frac{\theta}{2^{r-1}} - \tan\frac{\theta}{2^{r}}$$ After this, $$P(\theta)$$ becomes $$P(\theta) = \lim_{n \to \infty}\prod_{r=1}^n \frac{\sin\frac{\theta}{2^r}}{\tan\frac{\theta}{2^{r-1}}- \tan\frac{\theta}{2^r}}$$

One more detail I found out is that $$\lim_{n \to \infty} T(\theta,n) = 1$$, but I couldn't proceed further from here. Any hints/solutions are appreciated.

EDIT: After the hints in the comments, $$T(\theta, r)$$ resolves to $$\cos \frac \theta {2^{r-1}} \cos \frac \theta {2^r}$$ as follows (assuming $$\frac \theta {2^r} = t$$) $$\begin{gather} T(\theta, n) = \frac{\sin t}{\tan^2t\tan 2t + \tan t} \\ = \frac{\cos t}{\tan t \tan 2t + 1} \\ = \frac{\cos t(1-\tan^2t)}{1+\tan^2t} \\ = \cos t \cos 2t \\ = \cos \frac \theta {2^{r-1}} \cos \frac \theta {2^r} \end{gather}$$

Now, $$P(\theta) = \lim_{n \to \infty} \frac{ \left( \cos\theta\cos\frac\theta2... \cos \frac{\theta}{2^n} \right)^2 }{\cos\theta} = \frac{\sin^2\theta}{2^{2n}\sin^2 \frac \theta {2^n}\cos \theta} = \frac{\sin^2 \theta}{\theta^2 \cos \theta}$$ Therefore, $$\boxed{P(\pi/4) = \frac{8\sqrt2}{\pi^2}}$$

However, the answer mentioned in the textbook is $$\frac{2}{\pi}$$. Where am I going wrong? (I think there's a silly mistake somewhere here; just not able to find it :(

• The rth term in product is $\cos \frac{\theta} {2^{r-1}}\cos\frac {\theta} {2^r}$. Aug 10, 2020 at 5:22
• And you know that $\frac{\sin x} {x} =\prod_{r\geq 1}\cos(x/2^r)$. Aug 10, 2020 at 5:25
• The answer should come out $\frac{\sin \theta} {\theta} \frac{\sin 2\theta}{2\theta}$ which simplifies to $\frac{\sin^2\theta\cos\theta}{\theta^2}$ which equals $4\sqrt{2}/\pi^2$ if $\theta=\pi/4$. Aug 10, 2020 at 15:10
• The book answer is wrong by the way. Aug 10, 2020 at 15:11
• @ParamanandSingh You're right, I found the error. Thanks for helping me! (Can you post an answer so that I can accept it or should I go ahead and post one?) Aug 10, 2020 at 16:13

Carrying on from the answer: $$\begin{gather} T(\theta, n) = \frac{\sin t}{\tan^2t\tan 2t + \tan t} \\ = \frac{\cos t}{\tan t \tan 2t + 1} \\ = \frac{\cos t(1-\tan^2t)}{1+\tan^2t} \\ = \cos t \cos 2t \\ = \cos \frac \theta {2^{r-1}} \cos \frac \theta {2^r} \end{gather}$$
Now, $$P(\theta) = \cos\theta \cos \frac \theta 2 \cdot \cos\frac\theta2 \cos\frac\theta{2^2}\cdot ... = \left( \cos\theta \cos\frac\theta2...\right) \left( \cos\frac\theta2\cos\frac\theta{2^2}...\right)$$ Let $$\begin{gather} S = \lim_{n \to \infty}\cos\frac\theta2...\cos\frac\theta{2^n}\\ S\sin\frac\theta{2^n} = \lim_{n \to \infty} \frac{\sin\theta}{2^n} \\ S = \lim_{n \to \infty} \frac{\sin\theta}{2^n \sin\frac{\theta}{2^n}} = \lim_{t \to 0} \frac{t\sin\theta}{\sin(\theta t)}\\ S = \frac{\sin\theta}{\theta} \end{gather}$$
Therefore, $$P$$ reduces to $$P(\theta) = \frac{\sin2\theta\sin\theta}{2\theta^2}$$ And the value of $$P(\pi/4)$$ would be $$P(\pi/4) = \frac{16}{2\sqrt2\pi^2}\\ \boxed{P(\pi/4) = \frac{4\sqrt2}{\pi^2}}$$