# Vector Question Help

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#1
Hi guys, in this question, I don't get why they have subtracted Vector B from Vector A ?

Given that the point A has position vector –5i + 7j and the point B has position vector –8i + 2j
(a) Find the vector AB.
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1 year ago
#2
(Original post by lhh2003)
Hi guys, in this question, I don't get why they have subtracted Vector B from Vector A ?

Given that the point A has position vector –5i + 7j and the point B has position vector –8i + 2j
(a) Find the vector AB.
The other way round; vector A from vector B. And it's because of the following argument.

Suppose you start at the origin (from which every point is measured from) and you want to get to point B. Obviously, you can just travel along the vector OB which is -8i + 2j.

But you can also get to B via detour through A. So you can go to A first, then from A go to B. This is given in terms of vectors as OA + AB.

The result is the exact same displacement vector because both ways make you end up at B. Hence, OB = OA + AB.

And I'm sure you can see why AB = OB - OA now.
Last edited by RDKGames; 1 year ago
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#3
(Original post by RDKGames)
The other way round; vector A from vector B. And it's because of the following argument.

Suppose you start at the origin (from which every point is measured from) and you want to get to point B. Obviously, you can just travel along the vector OB which is -8i + 2j.

But you can also get to B via detour through A. So you can go to A first, then from A go to B. This is given in terms of vectors as OA + AB.

The result is the exact same displacement vector because both ways make you end up at B. Hence, OB = OA + AB.

And I'm sure you can see why AB = OB - OA now.
I don’t get why it wouldn’t be Ab average if the two.

Say Vector A = 4i + 5j , and Vector B = 5i + 7j.

Surely the resultant vector would be 5.5i + 6j ? It doesn’t make sense why the resultant vector would go out of the range of Ai to Bi for example.
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1 year ago
#4
(Original post by lhh2003)
I don’t get why it wouldn’t be Ab average if the two.

Say Vector A = 4i + 5j , and Vector B = 5i + 7j.

Surely the resultant vector would be 5.5i + 6j ? It doesn’t make sense why the resultant vector would go out of the range of Ai to Bi for example.
The resultant vector is what you get when you add up all of the vector. Not when take the average. I'm not sure where you're getting this idea from. You need to stick with the definitions for these things.
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