# Vector through two points

• November 30th 2009, 01:47 AM
Swlabr
Vector through two points
I know this isn't exactly "advanced" or "applied" maths, but I can't think where to put it...

How would I write down the vector through the two points $(x_1, y_1, z_1)$ and $(x_2, y_2, z_2)$?
• November 30th 2009, 01:48 AM
Moo
Hello,

Well, isn't it just $(x_2-x_1,y_2-y_1,z_2-z_1)$ ?
depending on the orientation of the vector... This one is if the vector starts at point 1 and goes to point 2.
• November 30th 2009, 01:52 AM
Swlabr
Quote:

Originally Posted by Moo
Hello,

Well, isn't it just $(x_2-x_1,y_2-y_1,z_2-z_1)$ ?
depending on the orientation of the vector... This one is if the vector starts at point 1 and goes to point 2.

Okay...my next question is then "What does this mean?" Are the points on the vector just scalar products of that vector? I.e. $(x_3, y_3, z_3) \in v$, the set of points on the vector, if there exists an $a$ such that $a(x_1-x_2) = x_3$, $a(y_1-y_2) = y_3$ and $a(z_1-z_2) = z_3$?

Because then, for instance, the vector through the points (1, 1, 2) and (2, 3, 4) is (1, 2, 2), and neither of the points is a scalar multiple of the vector coordinates...
• November 30th 2009, 01:57 AM
Moo
The vector calculates the variations between the two points.
Suppose you're moving along the vector. If you increase the abscissa by $x_2-x_1$, then the ordinate will be increased by $y_2-y_1$ and the third coordinate by $z_2-z_1$

If you want to describe the points that are moving along the line passing through the two points, then they're in the form $(x_1+a(x_2-x_1),y_1+a(y_2-y_1),z_1+a(z_2-z_1))$ (note the difference with what you noted)

I hope this is clear... Maybe a sketch would help ya
• November 30th 2009, 01:59 AM
Swlabr
Quote:

Originally Posted by Moo
The vector calculates the variations between the two points.
Suppose you're moving along the vector. If you increase the abscissa by $x_2-x_1$, then the ordinate will be increased by $y_2-y_1$ and the third coordinate by $z_2-z_1$

If you want to describe the points that are moving along the line passing through the two points, then they're in the form $(x_1+a(x_2-x_1),y_1+a(y_2-y_1),z_1+a(z_2-z_1))$ (note the difference with what you noted)

I hope this is clear... Maybe a sketch would help ya

No, that's perfect thanks!