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## #1 2013-12-01 23:35:29

pari_alf
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### direction of line segments

Hi,

I have three Points A(2,3) , B(3,1), C(9,3).
so i would have 2 line segments ab and bc.
I wanted to get the direction of line segment.
How could i get that.

## #2 2013-12-01 23:41:10

bobbym

Online

### Re: direction of line segments

Hi;

Which line segment? AC?

In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.

## #3 2013-12-02 00:12:06

pari_alf
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### Re: direction of line segments

yes, what i wanna do is.. if angel of segment ab is same as angle of bc
that meant points a ,b ,c are in the same direction.

then delete point b.. so we will consider line from point A to C.
so finally we should have a line AC with direction.

In other words we can say that point A , B , C are coplanar.

so to get the direction of segments, we need to get angle.

to get angle, i used this method

theta = acos ( (u.v) / |u| * |v| )

but this give angle of point with origin.

i don't need in that way.

Last edited by pari_alf (2013-12-02 00:22:10)

## #4 2013-12-02 05:39:58

bob bundy
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### Re: direction of line segments

hi pari_alf

Welcome to the forum.

You can use that formula but you need vectors that give the directions of the lines; eg

But, if all you want is to see if the points are co-linear, you could just work out the gradient of AB and the gradient of BC, to see if they are the same.

Bob

You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

## #5 2013-12-02 13:32:20

pari_alf
Member

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### Re: direction of line segments

#### bob bundy wrote:

hi pari_alf

Welcome to the forum.

You can use that formula but you need vectors that give the directions of the lines; eg

But, if all you want is to see if the points are co-linear, you could just work out the gradient of AB and the gradient of BC, to see if they are the same.

Bob

Thanks bob