You need to know the following important formulae and rules:[br][br][list][*]Distance between two points is found by Pythagoras' theorem: [math]\sqrt{\left(x_2-x_1\right)^2+\left(y_2-y_1\right)^2}[/math][/*][*]Coordinates of the midpoint of a line are the midpoints between the two coordinates:  [math]\left(\frac{x_1+x_2}{2},\frac{y_1+y_2}{2}\right)[/math] [/*][*]Gradient of a line is [b]change in y ÷ change in x[/b] (or [math]\frac{y_2-y_1}{x_2-x_1}[/math])[/*][*]If two lines are perpendicular, [math]m_1\times m_2=-1[/math][/*][*]The most commonly used equation of a straight line is [math]y-y_1=m\left(x-x_1\right)[/math][/*][*]You can also use [math]\frac{y-y_1}{y_2-y_1}=\frac{x-x_1}{x_2-x_1}[/math] if you have two known points.[/*][*]Points of intersection (where two lines cross) can be found using simultaneous equations.[/*][/list][br]Use the [i]Test Yourself[/i] below to practise finding distances, gradients, midpoints, perpendicular gradients and equations of straight lines.