What is the slope of a horizontal line?

Zero — the y-values do not change, so the numerator of the slope formula is zero.

Slope Formulas — Examples and Uses

The line formulas used most often in algebra and coordinate geometry.

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

Undefined when the x-values are equal (a vertical line).

$y = m x + b$

m is the slope, b is the y-intercept.

$y - y_{1} = m (x - x_{1})$

Use when you know one point and the slope.

$m_{1} m_{2} = - 1$

Perpendicular lines have negative reciprocal slopes.

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What is the slope of a horizontal line?: Zero — the y-values do not change, so the numerator of the slope formula is zero.