Example

Finding the Equation of a Line Perpendicular to y=4y = -4 Through (4,2)(-4, 2)

To find the equation of a line perpendicular to y=4y = -4 that passes through the point (4,2)(-4, 2), recognize that the given line is horizontal. Any line perpendicular to a horizontal line must be vertical, taking the form x=ax = a.

Since the perpendicular line is vertical and passes through (4,2)(-4, 2), every point on it shares the same x-coordinate of 4-4. Therefore, the equation of the perpendicular line is:

x=4x = -4

Note that vertical lines cannot be written in slope-intercept form because their slope is undefined. This example illustrates the companion special case to perpendiculars of vertical lines: when the given line is horizontal, the perpendicular is vertical and is determined entirely by the x-coordinate of the given point.

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Updated 2026-04-21

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