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Thread: determine angle of 3 points

  1. #1
    ave is offline
    May 2009

    determine angle of 3 points

    to explain the problem I will first paint a simple picture:

    imagine on the x/y axis I have a square who's center is exactly at the origin(0,0)

    I now want to work out the angles between the bottom right point and the rest, so bottom right and top right, bottom right and top left etc

    you will probably end up with something like 45 degrees, then 140, then 225

    I now want to do the same thing, but using 3d space ie the x,y,z axis, this time using a trapezium, still with its center lying on the origin

    I used the formula

    theta=Acos(dotproduct / sqrt(len)) and it works,

    here is the problem..... I cant remember what the technical term for this is, but if I apply the above formulae to the points [1,0,0] [0,1,0] meeting at [0,0,0] it gives me the answer 90 degrees, which is correct, but I want it to say 270 degrees- in other words measure it counter clockwise, not clockwise

    how would I go about doing this

    now I know normally I could just take 90 away from 360, but because im working with a plane in 3d space, and trying to get a computer program to work this out for me, if It spits out an answer of 90 degrees, im not sure if its worked it out in a clockwise or anticlockwise direction, and its really important that I know
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  2. #2
    Super Member malaygoel's Avatar
    May 2006
    Once you know the angle, you can check whether it is clockwise or anticlock-wise by applying sine product of vectors.
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