You should already be familiar with the equation of a straight line in a 2D plane as being $latex y = mx + c $. This section takes that work a step further. It allows us to express the equation of a straight line in 3D space in a similar format.

Take a straight line through the point A with direction vector $latex \mathbf{p} $. R is another point on that line. The position vector (from an arbitrary origin) of A is $latex \mathbf{a} $ , and of R is $latex \mathbf{r} $ . The vector $latex \overrightarrow{ \mathrm{AR} } $ is a multiple, $latex t $, of $latex \mathbf{p} $.

It follows that the position of a point R on the line is given by:

$latex \mathbf{r} = \overrightarrow{ \mathrm{OR} } = \overrightarrow{ \mathrm{OA} } + \overrightarrow{ \mathrm{AR} } = \mathbf{a} + t \mathbf{p} $

2 July 2013, Created with GeoGebra