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Math Help - Angle between two vectors

  1. #1
    Member Chokfull's Avatar
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    Angle between two vectors

    Im trying to find the angle between two vectors on Maple 13. I type in: "VectorAngle((3,7,7),(8,6,2))" and it gives me arccos(\frac{20}{1391}\sqrt{107}\sqrt{26}). This simplifies to .71 radians or about 40^o. The problem is the angle is supposed to be over 70 degrees from what I've worked out. Are my calculations wrong or did i type something in wrong in the calculator?

    I'm not sure if this is important, but when i typed in my vectors I, instead of the inside parentheses in red above, used some strange symbols that looked like < and > but taller and skinnier, but i don't know how to type these i had just copy-pasted them from somewhere else. When i copy-paste them here, though, they show up as <>, but they don't look the same when i try to type them into Maple as <>. I just don't get this at all
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  2. #2
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    Mathcad gave me the exact same result.
    Check your vectors.
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  3. #3
    A Plied Mathematician
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    There are ambiguities in the inverse cosine function. Another answer in the range of 0 to 360 degrees is 360 - 40 = 320 degrees. Does that answer make sense?
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    Member Chokfull's Avatar
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    Yeah-- \cos40^o=\cos 320^o is what you're saying, right? Either way, it's not the answer I'm looking for. The answer I want is about 70 degrees.
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    Member Chokfull's Avatar
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    Quote Originally Posted by Plato View Post
    Mathcad gave me the exact same result.
    Check your vectors.
    Mkay, thanks. I figured my calculations were probably what went wrong, but I wanted to ask here first. My vectors are right, and I'm fairly certain my 70 degrees is right, but i guess it's back to the drawing board. :P
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    Quote Originally Posted by Chokfull View Post
    Yeah-- \cos40^o=\cos 320^o is what you're saying, right? Either way, it's not the answer I'm looking for. The answer I want is about 70 degrees.
    You may well be looking for an angle of 70^o but with those two vectors your not going to find it.
    Again, check the vectors. Did you copy them correctly?
    Or could there be a different way of reading the problem?
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  7. #7
    Member Chokfull's Avatar
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    Yeah, like i said, I'm going to crunch all my numbers again. I'll probably post again if I don't find any mistakes in my math, but I'll betcha money I am.
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