Let the function f(x) = x3 + 3x + 3, x 0 be strictly increasing in (- , 1) ( 2, ) and strictly decreasing in ( 3, 4) ( 4, 5). Then i=15 i2 is equal to
- 48
- 40
- 36
- 28
Answer
(3) 36
Solution
f'(x) = 13 - 3x2 = x2 - 93x2
f'(x) > 0 when x2 > 9, i.e. x (- , -3) (3, ), so 1 = -3 and 2 = 3.
f'(x) < 0 when 0 < x2 < 9, i.e. x (-3, 0) (0, 3), so 3 = -3, 4 = 0 and 5 = 3.
i=15 i2 = 9 + 9 + 9 + 0 + 9 = 36
Answer: (3)