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. Please help!! Thanks!