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NDA 2026 · PYQ · Continuity and Differentiability · medium

Let f(x) = ax(x-1) for x < 1; x - 1 for 1 ≤ x ≤ 3; px² + qx + 2 for x > 3. Given that f(x) is continuous for all x but not differentiable at x = 1. Further f'(x) is continuous at x = 3. What is the value of q?

  1. A.-1✓ Correct
  2. B.-1/3
  3. C.1/3
  4. D.1

Explanation

From the conditions: continuity at x = 3 gives 9p + 3q + 2 = 2, so 9p + 3q = 0, hence q = -3p. Differentiability at x = 3 gives 6p + q = 1. Substituting q = -3p: 6p - 3p = 1, so p = 1/3, and q = -3(1/3) = -1.
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