A point on a curve is said to be an extremum if it is a local minimum or a local maximum. The number of distinct extrema for the curve 3x^4 - 16x^3 + 24x^2 - 37a is: A. 0 B. 1 C. 2 D. 3
GATE 2008 · Engineering Mathematics · Maxima Minima · easy
Answer: B. 1
- Differentiate and factor f'(x): f'(x) = 12x^3 - 48x^2 + 48x = 12x(x^2 - 4x + 4) = 12x(x - 2)^2. Setting f'(x) = 0: x = 0 or x = 2.
- Check second derivative at each critical point: f''(x) = 36x^2 - 96x + 48. f''(0) = 48 > 0 => x = 0 is a local minimum (extremum). f''(2) = 144 - 192 + 48 = 0 => test inconclusive for x = 2.
- Count extrema: Only x = 0 qualifies as an extremum. x = 2 is not. Total distinct extrema = 1.