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Thread: Multiple Angles like cos7x

  1. #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?

    Thanks already!

  2. #2
    Elite Member
    Join Date
    Jun 2007
    Posts
    12,531

    Re: Multiple Angles like cos7x

    Quote 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?

    Thanks already!
    Look at my response at:

    http://www.freemathhelp.com/forum/viewt ... 10&t=27719
    “... mathematics is only the art of saying the same thing in different words” - B. Russell

  3. #3
    Senior Member
    Join Date
    Aug 2007
    Posts
    1,303

    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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