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proof for Cos(A-B) using vectors.

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    #2
    **Timbuk2**
    Moderators, Education Moderators, Motoring & Transport Moderators Posts: 7,395 Mod ✭✭✭✭**Timbuk2**


    That's a nice proof!

    Not sure whether it would be allowed! To be on the safe side, I would probably just learn it the normal way. It's not that bad - just remember that you are getting the distance twice!


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    #3
    Evan93
    Registered Users Posts: 164 ✭✭Evan93


    As far as I know that proof is allowed. Our teacher said he would'nt do the other proof from the trig. chapter because the vector one is much easier to remember.


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    #4
    Fringe
    Registered Users Posts: 1,082 ✭✭✭Fringe


    The problem with that one is that the dot product is defined such that you use the cosine rule to show that it is equal to that. What you're doing is basically a circular proof. I think it is accepted though in the LC

    I like this one better though. The first one.
    http://en.wikipedia.org/wiki/Law_of_cosines


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    #5
    MathsManiac
    Registered Users Posts: 1,595 ✭✭✭MathsManiac


    This was discussed before:
    http://www.boards.ie/vbulletin/showthread.php?t=2055585026

    My view is, as it was then, that I doubt that the proof would be accepted as it stands.

    As Fringe has pointed out, various trigonometric identities are often used when you're proving the vector properties in the first place. If this formula or any that rely on it are used to do so then this proof would be a circular argument. You'd have to show, in my view, that the vector properties can be established without relying on the formula in question in order to make this proof satisfactory.

    If you want to be sure, why don't you e-mail the SEC and ask them?


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