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Math Help - Co-ordinate vector problem

  1. #1
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    Co-ordinate vector problem

    The question:
    Given the points A(2, -3, 1) and B(8, 9, -5) in R^3, find:

    The coordinates d of point D on the line AB such that D is between A and B and \vec{AD} = 2\vec{DB}

    My attempt:
    First I found the vector \vec{AB} by subtracting A from B to get (6, 12, -6). By drawing a small diagram, I realised I had to find the co-ordinate vector 2/3 the way to B from A. So I wrote the parametric form of a line with origin A (using the vector \vec{AB} that I calculated). I subbed in 2/3 as the scalar multiple, and produced (6, 5, -3) as the answer to this question.

    I checked this by finding \vec{AD} and equating it to 2\vec{DB} and both were equal to (4, 8, -4). I'm fairly sure my answer is correct, but my text has no solutions for practise exams, so I'd like to be sure.

    Thanks!
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  2. #2
    MHF Contributor

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    Yes, everything you did was correct. My only question is why, having analysed this problem, done the calculations, the checked and seen that your answer worked, why do you not have confidence in it?
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  3. #3
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    Because I often screw up, so there's no reason why my check wasn't screwed too. :P
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