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WAEC Further Mathematics 2007 Question 12

Question 12

Differentiation

The tangent to the curve y = 4x3 + kx2 − 6x + 4 at the point P(1, m) is parallel to the x-axis, where k and m are constants. Find the value of k.

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

Answer: D (−3)

Explanation

dy/dx = 12x² + 2kx − 6 must be 0 at x = 1: 12 + 2k − 6 = 0, so k = −3.