How would we prove this? I am familiar with deriving trig but not this. Can someone please shed some light and show me step by step on how to do this? Thank you.
Thank you for your help.
After I plugged that into the equation again the (cos a+b * cos a-b) cancels out and we are left with (-sin a + b sin a - b) - (sin a + b sin a - b) which should equal to -2 sin a + b sin a - b.
Now what would we do next? And what about the 2 in the denominator of the right hand side of the equation?