# Thread: Multiple Angles like cos7x

1. ## Multiple Angles like cos7x

Hello everyone,

I am helpless, because my teacher gave us a problem we don't have the tools to solve with (at least I cannot apply any of my tools)

how do I break this into smaller pieces: cos7x * cos13x = cosx * cos19x

do I have to use the double angle formula a whole bunch of times? Or is there another way to get the multiple angles split?

2. ## Re: Multiple Angles like cos7x

Originally Posted by rexexdesign
Hello everyone,

I am helpless, because my teacher gave us a problem we don't have the tools to solve with (at least I cannot apply any of my tools)

how do I break this into smaller pieces: cos7x * cos13x = cosx * cos19x

do I have to use the double angle formula a whole bunch of times? Or is there another way to get the multiple angles split?

Look at my response at:

http://www.freemathhelp.com/forum/viewt ... 10&t=27719

3. ## Re: Multiple Angles like cos7x

One approach would be to use the identity...

cos x cos y = (1/2)(cos(x+y) - cos(x-y)).

Or you might just look at it and wonder what happens if x=0.

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