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## #1 2013-12-01 00:35:29

pari_alf
Member
Registered: 2013-12-01
Posts: 85

### 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.

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## #2 2013-12-01 00:41:10

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: direction of line segments

Hi;

Which line segment? AC?

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #3 2013-12-01 01:12:06

pari_alf
Member
Registered: 2013-12-01
Posts: 85

### 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 (2013-12-01 01:22:10)

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## #4 2013-12-01 06:39:58

bob bundy
Registered: 2010-06-20
Posts: 8,322

### 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

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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## #5 2013-12-01 14:32:20

pari_alf
Member
Registered: 2013-12-01
Posts: 85

### 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

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