• Exploring the Concept of the Perpendicular Bisector

  • Deriving the Equation for the Perpendicular Bisector

  • Calculating the Midpoint of a Line Segment

  • Formulating the Equation of a Perpendicular Bisector

  • Conclusion

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Equation of a Perpendicular Bisector: How to Find/Calculate

In geometry, the perpendicular bisector holds a vital role, dividing a line segment into two equal parts while forming right angles with it. This fundamental concept not only underpins geometric constructions but also finds applications in various fields, from architecture to engineering.

Understanding its equation empowers us to analyze and solve geometric problems with precision and clarity. Throughout this journey, we will unravel the principles governing perpendicular bisectors, delve into their mathematical representations, and uncover the elegance of their equations. Join us as we embark on a quest to unlock the secrets of this geometric marvel.

Exploring the Concept of the Perpendicular Bisector

Delving into the world of geometry, the perpendicular bisector stands out as a critical notion. Let's dissect this term to understand its significance:

  • Perpendicular: This refers to lines intersecting at a precise right angle, or 90 degrees.
  • Bisector: This is characterized by the division of a line segment into two congruent parts.

In essence, a perpendicular bisector is a line that neatly divides another line into equal halves while intersecting it at a right angle.

Deriving the Equation for the Perpendicular Bisector

The equation of a perpendicular bisector emerges as a linear equation. Crafting this equation involves initially determining the slope's gradient for the bisector and then integrating known coordinates into a standard formula, such as \(y = mx + c\) or \(y - y_1 = m(x - x_1)\). Without the bisector's exact coordinate, calculating the line segment’s midpoint becomes essential.

Calculating the Bisector's Slope Gradient

  • The journey to formulating a perpendicular bisector's equation starts with determining the slope's gradient. Given that perpendicular slopes are inversely related, the gradient of the original line helps infer the bisector's gradient.
  • Expressed as -1 / m, the bisector's gradient inversely reciprocates the original line’s slope gradient, where m denotes the slope of the original line.

Considering line a defined by \(y = 3x + 6\) and bisected perpendicularly by line l, the quest is to uncover the gradient of line a:

  1. Determine the original gradient: With m representing the slope in \(y = mx + c\), the line's original gradient stands at 3.
  2. Calculate the perpendicular slope's gradient: Plugging the gradient 3 into the formula \(-\frac{1}{m}\) yields \(-\frac{1}{3}\) as the gradient of the bisector, given its perpendicular nature.

In scenarios devoid of the original line’s equation, deriving the slope's gradient necessitates using two coordinates with the formula \(\frac{y_2 - y_1}{x_2 - x_1}\).

Considering Line 1 extending from (3, 3) to (9, -21) and perpendicularly bisected by Line 2, the inquiry revolves around determining Line 2's slope gradient:

  1. Ascertain the original gradient: Lacking Line 1's equation mandates computing its slope gradient manually. Incorporate the given coordinates into the gradient formula \(\frac{change \, in \, y}{change \, in \, x}\), which simplifies to \(\frac{-24}{6} = -4\), upon substituting the numbers.
  2. Derive the bisector's gradient: Injecting -4 into \(-\frac{1}{m}\) respects the perpendicular relationship, resulting in \(\frac{1}{4}\) as the bisector's slope gradient.

Calculating the Midpoint of a Line Segment

The midpoint of a line segment is essentially the point that divides the segment into two equal parts. This calculation becomes particularly crucial when the original line's equation is unknown, and the perpendicular bisector's intersection point with the original line needs to be determined.

Simply put, a line segment connects two points along a line. The midpoint can be found by averaging the x and y coordinates of these endpoints. For example, the midpoint formula, given endpoints (a, b) and (c, d), is \(\left(\frac{a+c}{2}, \frac{b+d}{2}\right)\).

Consider a line segment with endpoints (-1, 8) and (15, 10). To find the midpoint coordinates:

  • Apply the formula \(\left(\frac{a+c}{2},\frac{b+d}{2}\right)\) with the given endpoints (-1, 8) and (15, 10), resulting in \(\left(\frac{-1+15}{2}, \frac{8+10}{2}\right) = (7, 9)\).

This formula can also be adapted to find one endpoint if the midpoint and the other endpoint are known.

For instance, if line segment AB has a midpoint of (6, 6) and endpoint A is at (10, 0), to find endpoint B:

  • Divide the midpoint formula into x and y components where the midpoint is (m, n):
    • For the x coordinate: \(\frac{a + c}{2} = m\)
    • For the y coordinate: \(\frac{b+d}{2} = n\)
  • Substitute the known coordinates accordingly:
    • For the x coordinate: \(\frac{10+c}{2} = 6\), leading to c = 2
    • For the y coordinate: \(\frac{0+d}{2} = 6\), leading to d = 12
  • Thus, endpoint B is located at the coordinates (2, 12).

Formulating the Equation of a Perpendicular Bisector

To derive the equation of a perpendicular bisector, integrate both the slope's gradient and the bisector's intersection point (midpoint) into an established linear equation.

Applicable formulas include:

  • \(y = mx + c\)
  • \(y - y_1 = m(x - x_1)\)
  • \(Ax + By = C\)

These formulas allow for direct substitution, except for the last, which requires reformatting.

Considering a line segment from (4,10) to (10, 20) bisected perpendicularly by line 1, the task is determining the perpendicular bisector's equation:

  1. Calculate the original line's slope gradient: \( \frac{20 - 10}{10 - 4} = \frac{5}{3} \).
  2. Determine line 1's slope gradient: By applying the inverse reciprocal of \(\frac{5}{3}\), we find it is \(-\frac{3}{5}\).
  3. Identify the midpoint: Calculating gives \( \left(\frac{4+10}{2}, \frac{10+20}{2}\right) = (7, 15) \).
  4. Formulate the equation: \( y - 15 = -\frac{3}{5}(x - 7) \), establishing the perpendicular bisector's equation.

The resulting equation for the line segment's perpendicular bisector is \( y - 15 = -\frac{3}{5}(x - 7) \).

For a segment from (-3, 7) to (6, 14) bisected perpendicularly by line 1, the steps to finding the perpendicular bisector's equation are:

  1. Calculate the original line's slope gradient: \( \frac{14-7}{6 - (-3)} = \frac{7}{9} \).
  2. Determine line 1's slope gradient: Applying the inverse reciprocal of \(\frac{7}{9}\), we obtain \(-\frac{9}{7}\).
  3. Find the midpoint: The calculation reveals \( \left(-\frac{3}{2}+6, \frac{7}{2}+14 \right) = \left(\frac{3}{2}, \frac{21}{2}\right) \).
  4. Formulate the equation: Substituting into \( y - \frac{21}{2} = -\frac{9}{7}(x - \frac{21}{2}) \) gives us the perpendicular bisector’s equation.

Thus, the equation for this line segment's perpendicular bisector is \( y - \frac{21}{2} = - \frac{9}{7}x + \frac{189}{14} \).


  • A perpendicular bisector divides a line segment evenly at a 90-degree angle, and is mathematically represented through a linear equation.
  • The slope of the perpendicular bisector is found by inversely reciprocating the original line's slope gradient, symbolized as -1 / m.
  • Without the original line's equation, ascertain the midpoint of the line segment—this crucial point where the bisector intersects. The midpoint formula is \(\left(\frac{a+c}{2},\frac{b+d}{2}\right)\), incorporating the endpoints of the line segment.
  • The final step in establishing the perpendicular bisector equation involves incorporating both the midpoint and the slope's gradient into a linear equation template.

This concise overview encapsulates the essence of perpendicular bisectors, showcasing their role in geometric analyses and the process for determining their equations.

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