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Math Help - Help with vectors...

  1. #1
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    Help with vectors...

    I have tried solving this again and again but I get wrong answer.

    Points A and B have coordinates (2,7) and (-3,-3) respectively. Use a vector method to find the coordinates of C and D, where

    (a) C is the point such that AC = 3AB (AC and AB have that arrow for vector notation)

    (b) D is the point such that AD = 3/5 AB ..

    I only need (a) and I hopefully that will help me understand it.

    Thanks
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  2. #2
    MHF Contributor
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    Quote Originally Posted by struck View Post
    I have tried solving this again and again but I get wrong answer.

    Points A and B have coordinates (2,7) and (-3,-3) respectively. Use a vector method to find the coordinates of C and D, where

    (a) C is the point such that AC = 3AB (AC and AB have that arrow for vector notation)

    (b) D is the point such that AD = 3/5 AB ..

    I only need (a) and I hopefully that will help me understand it.

    Thanks
    Hi

    A(2,7) and B(-3,-3)
    Therefore vector AB(-3-2,-3-7)
    AB(-5,-10)

    C is the point such that AC = 3AB
    AC(x-2,y-7)
    3AB(-15,-30)

    Therefore
    x-2=-15 and y-7=-30
    x=-13 and y=-23
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  3. #3
    Member Mentia's Avatar
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    so the vector AB is (-5, -10)

    If AC is 3AB then AC is (-15, -30)

    start at A and go down 30 left 15 and you get C = (2-15, 7-30) = (-13, -23)
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  4. #4
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    Dear struck,

    mark the origo O in koordinates (0, 0)
    So
    OA = (2,7)
    OB = (-3,-3)
    AB = OB - OA = (-3-2, -3-7) = (-5,-10)
    AC = 3*AB = (3*(-5),3*(-10)) = (-15,-30)

    If the question is OC than
    since AC = OC - OA --> OC = AC + OA = (-15+2,-30+7) = (-13,-23)
    The point C is (-13,-23).
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