Let f(x) = [x]2 - [x + 3] - 3, x R, where [.] is the greatest integer function. Then
- f(x) = 0 for finitely many values of x
- f(x) < 0 only for x [-1, 3)
- 02 f(x) dx = -6
- f(x) > 0 only for x [4, )
Answer
(2) f(x) < 0 only for x [-1, 3)
Solution
[x + 3] = [x] + 3
f(x) = [x]2 - [x] - 6 = ([x] - 3)([x] + 2)
(1) f(x) = 0 [x] = 3 or [x] = -2 x [3, 4) [-2, -1) ⇒ infinitely many values ⇒ false
(2) f(x) < 0 -2 < [x] < 3 [x] \-1, 0, 1, 2\ x [-1, 3) ⇒ true
(3) 02 f(x) dx = 01 (-6) dx + 12 (-6) dx = -12 ⇒ false
(4) f(x) > 0 [x] > 3 or [x] < -2 x (- , -2) [4, ) ⇒ not only [4, ) ⇒ false
Answer: (2)