<s>[INST] Find the value of $k$ in the equation $\sin {1^\circ}\sin {3^\circ}.......\sin {179^\circ} = \frac{1}{{{2^k}}}$ Find the value of 'k' in the equation $\sin {1^\circ}\sin {3^\circ}.......\sin {179^\circ} = \frac{1}{{{2^k}}}$
My approach is as follow
$\sin {1^\circ} = \sin {179^\circ}$
$T = {\sin ^2}{1^\circ}{\sin ^2}{3^\circ}..{\sin ^2}{89^\circ}$
$\left( {\frac{{1 - \cos {2^\circ}}}{2}} \right) = {\sin ^2}{1^\circ}$
$T = \left( {\frac{{1 - \cos {2^\circ}}}{2}} \right)\left( {\frac{{1 - \cos {6^\circ}}}{2}} \right)\left( {\frac{{1 - \cos {{10}^\circ}}}{2}} \right)....\left( {\frac{{1 - \cos {{178^\circ}}}}{2}} \right)$
$2 + \left( {n - 1} \right)4 = 178 \Rightarrow n = 45$
$T = \frac{1}{{{2^{45}}}}\left( {1 - \cos {2^\circ}} \right)\left( {1 - \cos {6^\circ}} \right)\left( {1 - \cos {{10}^\circ}} \right)....\left( {1 - \cos {{178^\circ}}} \right)$
$\left( {1 - \cos {{178}^\circ}} \right) = \left( {1 + \cos {2^\circ}} \right)$
$T = \frac{1}{{{2^{45}}}}\left( {1 - {{\cos }^2}{2^\circ}} \right)\left( {1 - {{\cos }^2}{6^\circ}} \right)\left( {1 - {{\cos }^2}{10^\circ}} \right)....\left( {1 - {{\cos }^2}{{86}^\circ}} \right)\left( {1 - \cos {{90}^\circ}} \right)$
$T = \frac{{{{\sin }^2}{2^\circ}.{{\sin }^2}{6^\circ}.{{\sin }^2}{{10}^\circ}......{{\sin }^2}{{86}^\circ}}}{{{2^{45}}}}$
Not able to proceed from here
 [/INST] As $\cos(90^\circ-A)=\sin A,$
$$\prod_{k=0}^{44}\sin(2k+1)^\circ=\prod_{k=0}^{44}\cos(2k+1)^\circ$$
As $\cos180(2k+1)^\circ=-1$
as $\cos180x=2^{180-1}c^{180}x+\cdots+(-1)^{90}$
and for $\cos180x=\cos180^\circ,180x=360^\circ n\pm180^\circ$
$x=(2 n+1)^\circ$  where $0\le n\le179$
If $\cos180x=-1,$  the roots of $$2^{180-1}c^{180}x+\cdots+(-1)^{90}=-1$$ are
$\cos(2 n+1)^\circ$ where $0\le n\le179$
$$\implies\prod_{n=0}^{179}\cos(2 n+1)^\circ=\dfrac2{2^{179}}$$
Now as $\cos(360^\circ-y)=+\cos y$
$$\implies\prod_{n=0}^{179}\cos(2 n+1)^\circ=\prod_{n=0}^{89}\cos^2(2 n+1)^\circ$$
Now $90<2n+1<180\iff 45\le n\le89\implies$ there are $45$(odd) number of multiplicands $<0$
$$\prod_{n=0}^{89}\cos(2 n+1)^\circ<0$$
$$\implies\prod_{n=0}^{89}\cos(2 n+1)^\circ=-\sqrt{\dfrac2{2^{179}}}$$
Finally as $\cos(180^\circ-z)=-\cos z,$
$$\implies\prod_{n=0}^{89}\cos(2 n+1)^\circ=-\prod_{n=0}^{44}\cos^2(2 n+1)^\circ$$
Finally use $$\prod_{n=0}^{44}\cos(2 n+1)^\circ>0$$
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