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JAMB Mathematics 2000 Question 36

Question 36

Differentiation

If y = 2x cos 2x − sin 2x, find dy/dx when x = π/4

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

Answer: B (−π)

Explanation

dy/dx = 2cos 2x − 4x sin 2x − 2cos 2x = −4x sin 2x. At x = π/4, sin(π/2) = 1, so dy/dx = −π.