A trigonometry exercise $2sin frac{pi}{14}+2sin frac{5pi}{14}-2sin frac{3pi}{14}=1$

How to prove that

$$2\sin \frac{\pi}{14}+2\sin \frac{5\pi}{14}-2\sin \frac{3\pi}{14}=1$$

I absolutely have no idea other than using the property
$\sin\bigl(\frac{\pi}{2}-a\bigr)=\cos a\;$