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WAEC Further Mathematics 2021 Question 21

Question 21

Dynamics

A stone is thrown vertically upwards and its distance, S metres, after t seconds is given by S = 12t + (5/2)t2 − t3. Calculate the maximum height reached.

  1. A.418.5 m
  2. B.56.0 m
  3. C.31.5 m
  4. D.24.0 m
Show the answer and explanation

Answer: C (31.5 m)

Explanation

dS/dt = 12 + 5t − 3t² = 0 gives (3t + 4)(t − 3) = 0, t = 3. S = 36 + 22.5 − 27 = 31.5 m.