The midpoint averages the coordinates. The distance is the Pythagorean length between them. Both show up on coordinate-plane questions. The section those belong to is coordinate geometry.
Midpoint
The midpoint of (x1, y1) and (x2, y2) is ((x1 + x2) / 2, (y1 + y2) / 2). The halfway point between (2, 4) and (8, 10) is (5, 7). If a question gives you the midpoint and one endpoint, plug in and solve for the missing endpoint. Do not subtract.
Distance
Distance is √[(x2 − x1)² + (y2 − y1)²]. That is the straight-line length, and it is the same relationship as the Pythagorean theorem. From (0, 0) to (3, 4), the distance is 5. If the choices are not simplified, match the radical you got before you decimalize it.
Both formulas are on the list in ACT Math formulas. Slope, which uses a difference but not a square, is in slope on ACT Math.
The bottom line
Average to find the middle. Square the differences and take a square root to find the length.
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