Hello every one,

it is about

$\displaystyle cos a + cos2a + cos 3a = \frac {sin (\frac{3a}{2})cos(\frac {3+1}{2}a)}{sin \frac {a}{2}}$

I am a bit embarrased to ask this cause I can go deep for hours in a real calculus problem and enjoy it, but unfortunately is not the same for trigonometry.

I just hate it, I find it so uninteresting and boring. I have looked at some tutorials already.. I always try to do my best before I post but this.. I cannot handle trigonometry that easy... call it psychological what ever...

Once in a while some exercise is asking for it and here I am unable to proove this equality.

Thank you all!