# Addition formulae - Trig help requested

1. Nov 9, 2006

### kalistella

addition formulae - Trig help requested!!

I have a problem with this with regard to Sin 120 or Cos 120.

Eg

sin 165
sin (120 +45)
sin120.cos45-cos120.sin45

How do I deal with sin 120 or cos 120?

I know that cos 45 is 1/square root 2 and so is sin 45.

But in the above problem, the answer comes to 1/4 (square root 6 - square root 2)

I don't get this answer.:uhh: My problem is with the 120 angles.

Thanks!

2. Nov 9, 2006

### arildno

Well, learn the correct addition formula properly.
Then, look at the unit circle what sin(120) and cos(120) should be.

3. Nov 11, 2006