Functions & Graphs

Straight-line graphs

The equation of a line is $y = mx + c$, where $m$ is the gradient and $c$ the y-intercept. $$\text{gradient} = \frac{\text{change in } y}{\text{change in } x} = \frac{y_2 - y_1}{x_2 - x_1}$$

Parallel lines have the same gradient.
Perpendicular lines have gradients whose product is $-1$ (the negative reciprocal): if $m_1=2$, then $m_2=-\frac{1}{2}$