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Math Help - Double and Half Angles

  1. #1
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    Double and Half Angles

    What is the purpose of these formulas?
    Why do we use them and what do they tell us?

    For example if CosA=5/9 what is Cos2A? what does 31/81 tell me?
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  2. #2
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    Quote Originally Posted by algebraisabeast View Post
    What is the purpose of these formulas?
    Why do we use them and what do they tell us?

    For example if CosA=5/9 what is Cos2A? what does 31/81 tell me?
    I'd use the double angle formula in cos: cos(2\theta) = 2cos^2\theta - 1
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  3. #3
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    I know the formula that I have to use. I know that since i have a value I can get the angle if I place that value in I can get the double angle. What is the point of the double angle? Why would I even look for this? Why do I need to be taught this?
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  4. #4
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    Quote Originally Posted by algebraisabeast View Post
    I know the formula that I have to use. I know that since i have a value I can get the angle if I place that value in I can get the double angle. What is the point of the double angle? Why would I even look for this? Why do I need to be taught this?
    HI

    if you were to find cos 60 without using the calculator , how would you do this ?

    Knowing the double angles would help a lot .

    cos 60 = cos 2(30)

    =2 cos^2 (30) -1

    =2(\frac{\sqrt{3}}{2})^2-1 and go on .

    Of course this is only one of its usage .
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