Find the point halfway along a line segment by averaging the x-coordinates and averaging the y-coordinates. It is on Foundation and Higher, aimed at grade 5. Free questions with answers are on this page. Worked examples show each line before you try the questions.
The midpoint of (x1,y1) and (x2,y2) is (2x1+x2,2y1+y2) — average the xs, average the ys. If M is the midpoint of A and B and you know A and M, B is as far again the other side: xB=2xM−xA.
A 'centre of a line segment' question on a geometry diagram is this formula, not a guess from counting squares (though counting squares is a fine check). Keep fractions: midpoint of (1,4) and (4,7) is (25,211), not a rounded (3,6).
Average the xs and average the ys. To find an endpoint from a midpoint, take the same jump again: from P to M is the vector, then M plus that vector is Q.
Objectives
Find the midpoint of two points.
Find only the x-coordinate or only the y-coordinate.
Find the distance between two points.
Key points
Find the midpoint of a segment from the coordinates of its two ends.
Cope with negative coordinates and answers that are decimals such as 2.5.
Work backwards: given the midpoint and one end, find the other end.
Use the midpoint idea in context, e.g. the centre of a rectangle from opposite corners.
Worked examples
Example 1
Work out the coordinates of the midpoint of (2, 6) and (8, 2).
2 marks
Add the x-coordinates and halve: (2 + 8) / 2 = 5
Add the y-coordinates and halve: (6 + 2) / 2 = 4
Write as a coordinate pair: (5, 4)
Answer: (5, 4)
Common slips
Slip: Archie did 2 + 8 = 10 without halving The midpoint requires finding the average of each coordinate.
Slip: Amber subtracted the x-coordinates: 8 - 2 = 6. Subtracting finds the gap between the points, not the middle. Add, then halve.
Slip: Archie wrote (10, 8) A midpoint must be halved.
Example 2
Work out the x-coordinate of the midpoint of (1, 8) and (7, 2).
2 marks
Identify the x-coordinates of the endpoints: 1 and 7
Calculate the average: (1 + 7) / 2 = 8 / 2 = 4
State the requested single coordinate: 4
Answer: 4
Common slips
Slip: Boris took the y-coordinates 8 and 2 The question asks specifically for the x-coordinate.
Slip: Bella did (8 + 2) / 2 = 5 Average only the x-coordinates.
Slip: Bella wrote (4, 5) Give only the x-coordinate as requested.