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JAMB Mathematics 1999 Question 23

Question 23

Coordinate Geometry

Find a positive value of α if the coordinate of the centre of a circle x2 + y2 − 2αx + 4y − α = 0 is (α, −2) and the radius is 4 units.

  1. A.1
  2. B.2
  3. C.3
  4. D.4
Show the answer and explanation

Answer: C (3)

Explanation

For this circle r² = α² + 2² + α = 16, so α² + α − 12 = 0, i.e. (α + 4)(α − 3) = 0. The positive value is 3.