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ESAT Maths 1 Coordinate geometry questions

1 original ESAT Maths 1 coordinate geometry questions in the real exam style, each with a step-by-step worked solution and an explanation of why the tempting wrong answers are wrong. Try each one before opening the solution.

Methods on this page: Perpendicular bisector: equidistant from both points

Question 1ESAT Maths 1Standard

The perpendicular bisector of the line segment joining A(4,4)A(4, 4) and B(−4,1)B(-4, 1) crosses the yy-axis at (0,k)(0, k). What is kk?
  1. A−52-\dfrac{5}{2}
  2. B12\dfrac{1}{2}
  3. C32\dfrac{3}{2}
  4. D52\dfrac{5}{2}
  5. E72\dfrac{7}{2}
  6. F92\dfrac{9}{2}
  7. G112\dfrac{11}{2}
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Answer: D · Method: Perpendicular bisector: equidistant from both points

  1. The midpoint of ABAB is (0,52)(0, \dfrac{5}{2}) and ABAB has gradient 38\dfrac{3}{8}, so the bisector has gradient −83-\dfrac{8}{3}.
  2. Alternatively: (0,k)(0, k) is the same distance from AA and BB.
  3. 16+(k−4)2=16+(k−1)216 + (k - 4)^2 = 16 + (k - 1)^2 gives k=52k = \dfrac{5}{2}.

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