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**pari_alf****Member**- Registered: 2013-12-01
- Posts: 40

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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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 83,230

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.**

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**pari_alf****Member**- Registered: 2013-12-01
- Posts: 40

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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**bob bundy****Moderator**- Registered: 2010-06-20
- Posts: 6,269

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

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**pari_alf****Member**- Registered: 2013-12-01
- Posts: 40

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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