## Cartesian equation of a line

Whilst it can be useful to express the equation of a line in its vector form $\mathbf{r} = \mathbf{a} + t \mathbf{p}$, it is also important that we are able to describe the line in a cartesian form, without the parameter t.

The expansion of the vector form gives the following general equation:
$\left( \begin{array}{c} x \\ y \\ z \end{array} \right) = \left( \begin{array}{c} a \\ b \\ c \end{array} \right) + t \left( \begin{array}{c} l \\ m \\ n \end{array} \right)$

If we rearrange these three equations for t we get:
$t = \frac{x - a}{l} \\ t = \frac {y - b} {m} \\ t = \frac {z - c} {n}$

Setting all these equal, as t is common to the equations, we get the following:

$\frac {x - a}{l} = \frac {y - b} {m} = \frac {z - c} {n}$