Coordinate Geometry: The Slopes of Perpendicular Lines
The slopes of two lines that are **perpendicular** to each other are negative reciprocals; in other words, if the slope of one line is $$m$$, then the slope of the other is $$-\frac{1}{m}$$.
In the above example the slope of one line is $$\frac{2}{5}$$, so the slope of the perpendicular line is the negative reciprocal: $$-\frac{1}{\frac{2}{5}} = -\frac{5}{2}$$.
In the above example the slope of one line is $$\frac{2}{5}$$, so the slope of the perpendicular line is the negative reciprocal: $$-\frac{1}{\frac{2}{5}} = -\frac{5}{2}$$.