ACT Math Review: Slope and Equations of Lines

m = \frac{y _{2} - y _{1}}{x _{2} - x _{1}}

Parallel lines have equal slopes. Perpendicular slopes are negative reciprocals.

Worked examples

$m = \frac{8 - 2}{4 - 1} = 2$

$m = 3$

$7 = 3 (1) + b$ , so $b = 4$

$y = 3 x + 4$

$m = - \frac{1}{2}$

$3 = - \frac{1}{2} (4) + b$ , so $b = 5$

$y = - \frac{1}{2} x + 5$

Using only the reciprocal for a perpendicular slope.

You also need to change the sign. The perpendicular slope to $2$ is $- \frac{1}{2}$ , not $\frac{1}{2}$ .

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