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# Scalar product question Watch

1. Hi, I was wondering how I would know the direction of the two lines which intersect when having to find the angle using the scalar product. I understand that both lines have to be facing the same way (either both outwards or both inwards).

Also, this is unrelated to the scalar product, but when doing b-a, is that to find the length between two lines? If so, why would I have to do the pythagoras theorem method on them? I don't understand what magnitude means... I thought it meant length... D:
Hi, I was wondering how I would know the direction of the two lines which intersect when having to find the angle using the scalar product. I understand that both lines have to be facing the same way (either both outwards or both inwards).

Also, this is unrelated to the scalar product, but when doing b-a, is that to find the length between two lines? If so, why would I have to do the pythagoras theorem method on them? I don't understand what magnitude means... I thought it meant length... D:
Magnitude is indeed length of a vector. Not sure what you mean in the first q
Hi, I was wondering how I would know the direction of the two lines which intersect when having to find the angle using the scalar product. I understand that both lines have to be facing the same way (either both outwards or both inwards).

Also, this is unrelated to the scalar product, but when doing b-a, is that to find the length between two lines? If so, why would I have to do the pythagoras theorem method on them? I don't understand what magnitude means... I thought it meant length... D:
You've really answered the first question yourself! If you have 3 points O, A and B then if X is the angle AOB you have

If you reversed both vectors then the magnitudes stay the same and , so you still get the same answer.

As long as you choose both vectors going in the same sense relative to point O then you will get the same answer.

For your second question, magnitude and length are the same thing for vectors so if A has position vector a and B has position vector b, then you can take the magnitude of either a-b or b-a to get the distance between them.

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Updated: April 17, 2013
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