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Math Help - Double Angle Formulas

  1. #1
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    Double Angle Formulas

    Hello, i am in a grade 12 advanced functions class (i guess it's pretty much a pre-calc class) and we're doing double angle formulas and i don't get it at all. Teacher isn't a calculus teacher so she can't explain it very well. Anywho, my question is:

    Use a double angle formula to rewrite:
    cos2(2x)

    What i did was just times it out and got:
    cos4x

    But i don't know what i'm doing....

    please help.
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  2. #2
    is up to his old tricks again! Jhevon's Avatar
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    Quote Originally Posted by CenturionMonkey View Post
    Hello, i am in a grade 12 advanced functions class (i guess it's pretty much a pre-calc class) and we're doing double angle formulas and i don't get it at all. Teacher isn't a calculus teacher so she can't explain it very well. Anywho, my question is:

    Use a double angle formula to rewrite:
    cos2(2x)

    What i did was just times it out and got:
    cos4x

    But i don't know what i'm doing....

    please help.
    that rewrite is not very enlightening is it. where have you sued the double angle formula?

    recall, the double angle formula for cosine says: \cos 2A = \cos^2 A - \sin^2 A = 2 \cos^2 A - 1 = 1 - 2 \sin^2 A

    thus, \cos 2(2x) = \cos^2 2x - \sin^2 2x = 2 \cos^2 2x - 1

    and you can go even further by expanding the \cos 2x
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  3. #3
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    Hey yeah thanks, i actually figured it out. I was overthinking it. I thought i had to find the angle or something. I don't know what i was thinking. lol. Thanks for the help anyway.
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