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Math Help - Finding the angle between two vectors

  1. #1
    Junior Member
    Joined
    Aug 2009
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    37

    Arrow Finding the angle between two vectors

    I think I have made an error somewhere here as you cannot take the square of a negative in real terms. See below.

    U = (-1, -1, -1), V = (3, 1, 0).

    U . V = (-1, -1, -1) . (3, 1, 0) = ( (-3) + (-1) + 0 ) = -4.

    ||U|| SQRT( (-1)^2 + (-1)^2 + (-1)^2 ) = SQRT( (-3) ).

    ||V|| SQRT( 3^2 + 1^2 + 0^2 ) = SQRT( 10 ).

    Cos THETA = U . V / ||U|| ||V|| = -4 / SQRT((-3)) * SQRT(10)

    THETA Cos^-1 ( -4 / SQRT((-3)) * SQRT(10))

    I am thinking this angle is obtuse.

    I'm really sorry for the presentation of this but I'm not sure how the tags work. If it's too much of a mess I can try and work out how to use the tags.

    Thanks,
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  2. #2
    Senior Member
    Joined
    Jul 2009
    Posts
    397
    Hi RedKMan

    Here is the error :
    ||U|| SQRT( (-1)^2 + (-1)^2 + (-1)^2 ) = SQRT( (-3) )
    (-1)^2 = 1
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