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Math Help - cos-1

  1. #1
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    cos-1

    I try to find X from the cosine formula but it doesn't sound right

    12sq = 8 sq + 5sq - 2x 8x 5cosX
    144 = 64 + 25 - 80cosX
    55 = -80cosX
    x= cos^-1(-55/80)

    I keep getting 2.328837092
    Please help
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  2. #2
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    Well, when you get the correct answer, you should quit. Why are you still streesing over it?

    Would you be happier with \pi - acos\left(\frac{11}{16}\right)

    Maybe you were expecting the answer in degrees? 133.433 deg
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  3. #3
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    how did you get 133.433 deg?

    "In mathematics, you don't understand things. You just get used to them." -- Johann von Neumann
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  4. #4
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    Quote Originally Posted by terminator View Post
    how did you get 133.433 deg?
    using degree mode in your calculator ...

    \displaystyle 180 - \arccos\left(\frac{11}{16}\right) \approx 133^\circ
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  5. #5
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    Quote Originally Posted by terminator View Post
    I try to find X from the cosine formula but it doesn't sound right

    12sq = 8 sq + 5sq - 2x 8x 5cosX
    144 = 64 + 25 - 80cosX
    55 = -80cosX
    x= cos^-1(-55/80)

    I keep getting 2.328837092
    Please help
    If you are thinking about x as being an angle ("cosine" is a function- just because you talk about cos(x) it does not necessarily mean that x is and angle) then x= 2.3288... radians.

    Since \pi= 3.14159... radians corresponds to 180 degrees, 2.3288 radians= 2.3288 (180/3.14159)= 133.433... degrees.
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