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Math Help - subtracting one point from another

  1. #1
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    subtracting one point from another

    Hi,

    I think this is an easy question, but I'm obviously forgetting something.

    There are two points in the first quadrant. The question is in the form of a picture (no numbers), but the points look something like A=(2,4); B=(5,1). Then it asks where point C would be if C=A-B.

    I thought the answer would be C = (XsubA-XsubB , YsubA - YsubB) = (2-5 , 4-1) = (-3,3), which would place it in the second quadrant. But the answer key has it in the fourth quadrant. What am I doing wrong?

    Thanks!
    Murph
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  2. #2
    Bar0n janvdl's Avatar
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    Quote Originally Posted by murph View Post
    Hi,

    I think this is an easy question, but I'm obviously forgetting something.

    There are two points in the first quadrant. The question is in the form of a picture (no numbers), but the points look something like A=(2,4); B=(5,1). Then it asks where point C would be if C=A-B.

    I thought the answer would be C = (XsubA-XsubB , YsubA - YsubB) = (2-5 , 4-1) = (-3,3), which would place it in the second quadrant. But the answer key has it in the fourth quadrant. What am I doing wrong?

    Thanks!
    Murph
    Why not say (5 - 2 ; 1 - 4) = (3 ; -3)

    That would put it into the fourth quadrant.

    But then it would be B - A...

    No, you're right then. Then it should be in the second quadrant.
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  3. #3
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    Thanks for the reply!

    So does that mean that in general, C=A-B would be calculated by

    C = (XsubB-XsubA, YsubB - YsubA)

    rather than

    C = (XsubA-XsubB , YsubA-YsubB)

    It's hard for me to understand why it would work that way. Or was there just a typo in the question (or answer key) that I was given?
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  4. #4
    Bar0n janvdl's Avatar
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    Quote Originally Posted by murph View Post
    Thanks for the reply!

    So does that mean that in general, C=A-B would be calculated by

    C = (XsubB-XsubA, YsubB - YsubA)

    rather than

    C = (XsubA-XsubB , YsubA-YsubB)

    It's hard for me to understand why it would work that way. Or was there just a typo in the question (or answer key) that I was given?
    No, you were right, it should be in the 2nd quadrant. I think the book is wrong. It should have been C=B-A then.
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