Let x = -1 and x = 2 be the critical points of the function f(x) = x3 + ax2 + b e |x| + 1, x 0. Let m and M respectively be the absolute minimum and the absolute maximum values of f in the interval [-2, - 12 ]. Then |M + m| is equal to (Take e 2 = 0.7):
- 21.1
- 19.8
- 22.1
- 20.9
Answer
(1) 21.1
Solution
f'(x) = 3x2 + 2ax + bx
f'(-1) = 0 3 - 2a - b = 0 ... (i)
f'(2) = 0 12 + 4a + b2 = 0 8a + b = -24 ... (ii)
Adding (i) and (ii): 6a + 3 = -24 a = - 92, b = 3 - 2a = 12
f(x) = x3 - 92x2 + 12 e |x| + 1
In [-2, - 12 ] the only critical point is x = -1.
f(-2) = -8 - 18 + 12 e 2 + 1 = -25 + 8.4 = -16.6
f(-1) = -1 - 92 + 0 + 1 = -4.5
f (- 12 ) = - 18 - 98 - 12 e 2 + 1 = -1.25 - 8.4 + 1 = -8.65
M = -4.5, m = -16.6
|M + m| = |-21.1| = 21.1
Answer: (1)