Lesson Objectives
  • Learn how to find the midpoint of a line segment

How to Find the Midpoint of a Line Segment


In this lesson, we want to discuss the midpoint formula. First and foremost, let’s introduce the concept of a line segment. A line segment is just a piece of a line. Unlike a line, it has two endpoints and a defined length. Now, the midpoint is just the point that is equidistant (meaning it has the same distance) from the endpoints of our line segment. In other words, the midpoint will cut the line segment in half. Suppose we have a line segment with endpoints: $$(x_1, y_1), (x_2, y_2)$$ showing a line segment graphically We have plotted the point (x,y) as the midpoint of our line segment. To find the x-value, we know that the distance from x1 to x is the same as the distance from x2 to x. $$x_2 - x=x - x_1$$ Solve for x: $$x=\frac{x_1 + x_2}{2}$$ Notice how we are just finding the average of the x-coordinates from our endpoints. We can do the same thing for y: $$y_2 - y=y - y_1$$ Solve for y: $$y=\frac{y_1 + y_2}{2}$$ Again, we are just finding the average of the y-coordinates from our endpoints. We will use a capital M to denote the midpoint, recall that a lowercase m is used for slope. $$M=\left(\frac{x_1 + x_2}{2}, \frac{y_1 + y_2}{2}\right)$$ Example 1: Find the midpoint of the line segment with the given endpoints. $$(9, 3), (2, -1)$$ $$M=\left(\frac{9 + 2}{2}, \frac{3 - 1}{2}\right)$$ $$M=\left(\frac{11}{2}, 1\right)$$

Skills Check:

Example #1

Find the midpoint of the line segment PQ. $$P: (3, 1), Q: (9, -5)$$

Please choose the best answer.

A
$$(1, -3)$$
B
$$(6, -2)$$
C
$$(6, 2)$$
D
$$(-1, 3)$$
E
$$(4, 6)$$

Example #2

Find the midpoint of the line segment PQ. $$P: (1, -9), Q: (3, -12)$$

Please choose the best answer.

A
$$\left(2, -\frac{21}{2}\right)$$
B
$$\left(5, -\frac{1}{2}\right)$$
C
$$\left(6, -3\right)$$
D
$$\left(-1, \frac{21}{2}\right)$$
E
$$\left(\frac{21}{2}, 2\right)$$

Example #3

Find the midpoint of the line segment PQ. $$P: (-15, 2), Q: (5, 6)$$

Please choose the best answer.

A
$$(4, -5)$$
B
$$(-5, 4)$$
C
$$(5, -4)$$
D
$$(3, 2)$$
E
$$(-1, 2)$$
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