Example

Finding the Equation of a Vertical Line Through (3,5)(-3, 5) and (3,4)(-3, 4)

To find the equation of a line containing the points (3,5)(-3, 5) and (3,4)(-3, 4), first attempt to calculate its slope using the slope formula. Step 11 — Find the slope. m=453(3)m = \frac{4 - 5}{-3 - (-3)} Simplify the denominator by turning the subtraction of a negative into addition (3+3-3 + 3): m=10m = \frac{-1}{0} Because division by zero is mathematically undefined, the slope is undefined. An undefined slope always indicates that the line is vertical. Step 22 — Write the equation. Because both given points have an xx-coordinate of 3-3, every point on this vertical line shares that exact xx-coordinate. Therefore, the equation of the line is: x=3x = -3 Because there is no yy variable, this equation cannot be written in slope-intercept form. Encountering an undefined slope during the two-points procedure is a direct cue to write a vertical line equation x=ax = a.

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Updated 2026-05-03

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