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    sdlinsey's Avatar
    sdlinsey Posts: 12, Reputation: 1
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    #1

    Apr 26, 2009, 08:28 PM
    Write the expression as the sin, cos, or tan of a double angle. Then find the exact
    Write the expression as the sine, cosine, or tangent of a double angle. Then find the exact value of the expression

    I got it to 2 cos2 (45+30) -1 usingthe addition and subraction formula but I'm lost from there

    2 cos2(75degrees) - 1
    Perito's Avatar
    Perito Posts: 3,139, Reputation: 150
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    #2

    Apr 26, 2009, 08:57 PM

    What is the original question?
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    sdlinsey Posts: 12, Reputation: 1
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    #3

    Apr 26, 2009, 09:00 PM
    the original question was Write the expression as the sine, cosine, or tangent of a double angle. Then find the exact value of the expression
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    Perito Posts: 3,139, Reputation: 150
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    #4

    Apr 26, 2009, 09:03 PM

    What was the expression? Surely it's wasn't "2 cos2 (45+30) -1", was it?
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    sdlinsey Posts: 12, Reputation: 1
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    #5

    Apr 26, 2009, 09:06 PM
    No it was 2 cos2(75 degrees) - 1
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    Perito Posts: 3,139, Reputation: 150
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    #6

    Apr 26, 2009, 09:19 PM



    I don't get it. You can actually solve that. There's no need for a closed form solution. OK. Enough rant...

    Let's look at where θ = 75 degrees and see where that gets us.

    There is a trig identity that says



    Using that, we can write your expression as



    or




    That should do it.
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