Finding the value of $\prod^{6}_{r=0}\cos\bigg(\frac{\pi}{21}+\frac{r\pi}{7}\bigg)$ 
Find value of $$\prod^{6}_{r=0}\cos\left(\frac{\pi}{21}+\frac{r\pi}{7}\right)$$

What I try:
$$\cos \frac{\pi}{21}\cdot \cos \frac{4\pi}{21}\cdot \cos\frac{7\pi}{21}\cdot \cos \frac{10\pi}{21}\cdot \cos \frac{13\pi}{21}\cdot \cos \frac{16\pi}{21}\cdot \cos \frac{19\pi}{21}$$
$\displaystyle \cos \frac{19\pi}{21} = -\cos \frac{2\pi}{21}$ and $\displaystyle \cos \frac{13\pi}{21}=-\cos \frac{8\pi}{21}$
How do I solve it? Help me, please.

I believe this is how the problem came into being
Use $\cos(\pi-x)=-\cos x,$
$P=-\prod_{r=0}^6\cos(\pi/21+2r\pi/7),$
Observe that $\cos7(x+2r\pi/7)=\cos7x,$
Now if $\cos(2n+1)y=\cos(2n+1)x$ which is $2^{2n}\cos^{2n+1}x+\cdots+(-1)^n(2n+1)\cos x$
So, the roots of $$2^{2n}\cos^{2n+1}x+\cdots+(-1)^n(2n+1)\cos x-\cos(2n+1)y=0$$ are $\cos(2r\pi/(2n+1)+y)$ where $0\le r\le2n$
So, the product of the roots will be $$\dfrac{\cos7y}{2^{2n}}$$
Here $7y=\pi/3$ and $2n+1=7\iff n=?$
