Simplify sin(A) + sin(A + 2π/3) + sin(A + 4π/3)

[Silver Bolt]

$\sin\left({A}\right)+\sin\left({A+\frac{2\pi}{3}}\right)+\sin\left({A+\frac{4\pi}{3}}\right)=0$

$L.H.S=\sin\left({A}\right)+\left(\sin\left({A}\right)\cos\left({\frac{2\pi}{3}}\right)+\cos\left({A}\right)\sin\left({\frac{2\pi}{3}}\right)\right)+\left(\sin\left({A}\right)\cos\left({\frac{4\pi}{3}}\right)+\cos\left({A}\right)\sin\left({\frac{4\pi}{3}}\right)\right) $

From there?

 

[MarkFL]

You've only given an expression...what is the actual identity to be verified?

 

[Silver Bolt]

Corrected now

 

[MarkFL]

$\sin\left({A}\right)+\sin\left({A+\frac{2\pi}{3}}\right)+\sin\left({A+\frac{4\pi}{3}}\right)=0$

$L.H.S=\sin\left({A}\right)+\left(\sin\left({A}\right)\cos\left({\frac{2\pi}{3}}\right)+\cos\left({A}\right)\sin\left({\frac{2\pi}{3}}\right)\right)+\left(\sin\left({A}\right)\cos\left({\frac{4\pi}{3}}\right)+\cos\left({A}\right)\sin\left({\frac{4\pi}{3}}\right)\right) $

From there?

I would write the LHS as:

$$\sin(A)\left(1+\cos\left(\frac{2\pi}{3}\right)+\cos\left(\frac{4\pi}{3}\right)\right)+\cos(A)\left(\sin\left(\frac{2\pi}{3}\right)+\sin\left(\frac{4\pi}{3}\right)\right)$$

Now, what are:

$$1+\cos\left(\frac{2\pi}{3}\right)+\cos\left(\frac{4\pi}{3}\right)=?$$

$$\sin\left(\frac{2\pi}{3}\right)+\sin\left(\frac{4\pi}{3}\right)=?$$