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:

Taking

=

=

This reduces to

Now I'm stuck.

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- Aug 29th 2008, 10:56 AMPersaGellTrigonometric Identity
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:

Taking

=

=

This reduces to

Now I'm stuck. - Aug 29th 2008, 11:07 AMMoo
Hi !

(There's the red part that was missing :))

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) - Aug 29th 2008, 02:35 PMThePerfectHacker
It is even easier.

Use complex numbers and geometric series. - Aug 29th 2008, 06:01 PMShyam
Your last line is wrong, which is

See here,