Question 1
2 marks\(A\) is the point \((-6.4, 3.8)\).
\(M\) is the point \((1.2, -0.5)\).
\(M\) is the midpoint of the line segment \(AB\).
Find the coordinates of \(B\).
Model answer
From \(A\) to \(M\): x goes up by \(1.2 - (-6.4) = 7.6\), y goes down by \(3.8 - (-0.5) = 4.3\)
From \(M\) to \(B\) is the same step again:
x: \(1.2 + 7.6 = 8.8\)
y: \(-0.5 - 4.3 = -4.8\)
\(B = (8.8, -4.8)\)
Explanation
\(M\) is halfway from \(A\) to \(B\), so the step from \(M\) to \(B\) is the same as the step from \(A\) to \(M\). Equivalently, \(B = 2M - A\) for each coordinate. A common mistake is to find the midpoint of \(A\) and \(M\) instead.
Mark scheme
-
1 mark
M1 for a correct method for one coordinate, e.g. \(2 \times 1.2 - (-6.4)\) or \(1.2 + 7.6\), or \(2 \times (-0.5) - 3.8\) or \(-0.5 - 4.3\)
oe, e.g. \(\dfrac{-6.4 + x}{2} = 1.2\); correct answer with no working scores full marks
-
1 mark
A1 for (8.8, −4.8)
cao