# Thread: subtracting one point from another

1. ## 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

2. Originally Posted by murph
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.

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?

4. Originally Posted by murph