Let a > 0. If the function f(x) = 6x3 - 45ax2 + 108a2x + 1 attains its local maximum and minimum values at the points x1 and x2 respectively such that x1 x2 = 54, then a + x1 + x2 is equal to :
- 15
- 13
- 24
- 18
Answer
(4) 18
Solution
f'(x) = 18x2 - 90ax + 108a2 = 18(x2 - 5ax + 6a2) = 18(x - 2a)(x - 3a)
f'(x) = 0 x = 2a, 3a
f''(x) = 36x - 90a: f''(2a) = -18a < 0 (maximum), f''(3a) = 18a > 0 (minimum)
x1 = 2a, x2 = 3a
x1 x2 = 6a2 = 54 a2 = 9 a = 3 ( a > 0)
x1 = 6, x2 = 9
a + x1 + x2 = 3 + 6 + 9 = 18
Answer: (4)