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JAMB Mathematics 2005 Question 38

Question 38

Differentiation

The radius r of a circular disc is increasing at the rate of 0.5cm/sec. At what rate is the area of the disc increasing when its radius is 6cm?

  1. A.36π cm2/sec.
  2. B.18π cm2/sec.
  3. C.6π cm2/sec.
  4. D.3π cm2/sec.
Show the answer and explanation

Answer: C (6π cm2/sec.)

Explanation

A = πr2, so dA/dt = 2πr × dr/dt = 2π × 6 × 0.5 = 6π cm2/sec.