<s>[INST] Evaluating $\sin\frac{\pi}{2}\sin\frac{\pi}{2^2}\sin\frac{\pi}{2^3}\cdots\sin\frac{\pi}{2^{11}}\cos \frac{\pi}{2^{12}}$ Evaluate :
$$\sin\frac{\pi}{2} \times \sin \frac{\pi}{2^2} \times \sin \frac{\pi}{2^3} \cdots \times \sin\frac{\pi}{2^{11}} \times \cos \frac{\pi}{2^{12}}$$
I tried to solve it by using double angle formula, by replacing $\sin\displaystyle\frac{\pi}2$ with   $2 \sin\displaystyle\frac{\pi}4 \cos\displaystyle\frac{\pi}4$ and similarly replacing $\sin\displaystyle\frac{\pi}{2^2}$ with double angle formula. But I'm not getting anywhere from this, can you please suggest me how can I approach to this problem, especially that cosine term at last is making everything so weird.
The thing which is most important to ask is : although I know every theory which applies over here but then also why I'm unable to solve this question? What is the reason for my failure? 
 [/INST] Whatever you would do, the result would be a very small number very close to $0$
$$\sin\frac{\pi}{2} \times \sin \frac{\pi}{2^2} \times \sin \frac{\pi}{2^3}\times \cdots \times \sin\frac{\pi}{2^{11}} \times \cos \frac{\pi}{2^{12}}$$
We know the value of the sine for some of the angles. The first are
$$\{1 , \frac {\sqrt 2} 2,\frac{\sqrt{2-\sqrt{2}}}{2},\frac{\sqrt{2-\sqrt{2+\sqrt{2}}}}{2},\frac{\sqrt{2-\sqrt{2+\sqrt{2+\sqrt{2}}}}}{2},\frac{\sqrt{2-\sqrt{2+\sqrt{2+\sqrt{2+\sqrt{2}}}}} }{2}  \}$$ and the pattern continue for ever.
For the next one, corresponding to $\sin \frac{\pi}{2^8}$ the exact value is
$$\frac{\sqrt{2-\sqrt{2+\sqrt{2+\sqrt{2+\sqrt{2+\sqrt{2+\sqrt{2}}}}}}}}{2}\approx0.012271538$$ while
$$\frac{\pi}{2^8} \approx 0.012271846$$ that is to say that you can approximate all the sines by the argument. Concerning the cosine, the argument is so small that you can assume it is almost equal to $1$.
If you take into account the fact that $\sin(x)<x$, an upper bound for the product of sines would be
$$ \frac{ \pi^9 \,\sqrt 2}{2^{64}}\approx 2.285 \times 10^{-15}$$ while it should be $\approx 2.208 \times 10^{-15}$.
Concerning
$$\cos \frac{\pi}{2^{12}}\approx 0.99999970586$$
All of that makes the value of the expression quite close to the machine epsilon.
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