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WAEC Further Mathematics 2020 Question 28

Question 28

Functions

A function f defined by f: x → x2 + px + q is such that f(3) = 6 and f′(3) = 0. Find the value of q.

  1. A.−9
  2. B.−6
  3. C.15
  4. D.21
Show the answer and explanation

Answer: C (15)

Explanation

f′(x) = 2x + p, so f′(3) = 0 gives p = −6. Then f(3) = 9 − 18 + q = 6, so q = 15.