Let f : [-1, 2] R be given by f(x) = 2x2 + x + [x2 ] - [x], where [t] denotes the greatest integer less than or equal to t. The number of points, where f is not continuous, is :
- 5
- 6
- 4
- 3
Answer
(3) 4
Solution
2x2 + x is continuous check only where x2 or x is an integer: x = -1, 0, 1, 2, 3, 2
x = -1: f(-1) = 2 - 1 + 1 + 1 = 3, f(-1+) = 2 - 1 + 0 + 1 = 2 discontinuous
x = 0: f(0-) = 0 + 0 + 0 + 1 = 1, f(0) = 0 discontinuous
x = 1: f(1-) = 3 + 0 - 0 = 3, f(1) = f(1+) = 3 + 1 - 1 = 3 continuous (both jumps cancel)
x = 2, 3: only [x2] jumps discontinuous
x = 2: f(2-) = 10 + 3 - 1 = 12 = f(2) = 10 + 4 - 2 continuous
4 points: -1, 0, 2, 3
Answer: (3)