Hi, I need help on this problem:
Use the identity and the telescoping property of finite sums to prove that if (m an integer), we have
Here is what I have so far:
This reduces to
Now I'm stuck.
Once you're here, try to think "how can I transform a difference of sines into a product of a sine and a cosine ?" and then use this formula (which is quite the same as the identity you're given at the very beginning) :
And you're done
(this formula is available, as well as many others, here : Trigonometry)