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When the interval is not parallel to one of the axes we take the average of the x-coordinates and the y-coordinates.

Cartesian plane.
Detailed description

Example 2

Let M be the midpoint of the line AB. Triangles AMS and MBT are congruent triangles (AAS), and so \(AS = MT\) and \(MS = BT\).

Hence the x-coordinate of M is the average of 1 and 5.

\(x = \dfrac{5 + 1}{2} = 3\)

The y-coordinate of M is the average of 2 and 8.

\(x = \dfrac{2 + 8}{2} = 5\)

Thus the coordinates of the midpoint M are (3, 5).