Equation of Straight Line in Plane

Theorem
A straight line $L$ is the set of all $\left({x, y}\right) \in \R^2$, where:
 * $\alpha_1 x + \alpha_2 y = \beta$

where $\alpha_1, \alpha_2, \beta \in \R$ are given, and not both $\alpha_1, \alpha_2$ are zero.

Proof
Let $y = f \left({x}\right)$ be the equation of a straight line.

A straight line has constant gradient.

Thus the derivative of $y$ w.r.t. $x$ will be of the form:
 * $y' = c$

Thus:

where $K$ is arbitrary.

Taking the equation:
 * $\alpha_1 x + \alpha_2 y = \beta$

it can be seen that this can be expressed as:
 * $y = - \dfrac {\alpha_1}{\alpha_2} x + \dfrac {\beta} {\alpha_2}$

thus demonstrating that $\alpha_1 x + \alpha_2 y = \beta$ is of the form $y = c x + K$ for some $c, K \in \R$.