suppose f is continuous on [0,1], f(x) is greater than or equal to 0 and f(1/2) is greater than 0.

a. prove that the integral of f(x) dx from 0 to 1 is greater than 0.

b. show, by counterexample, that the assumption of continuity is necessary.

Results 1 to 2 of 2

- Jun 1st 2006, 04:16 PM #1

- Joined
- Jun 2006
- Posts
- 53

## cal problem

suppose f is continuous on [0,1], f(x) is greater than or equal to 0 and f(1/2) is greater than 0.

a. prove that the integral of f(x) dx from 0 to 1 is greater than 0.

b. show, by counterexample, that the assumption of continuity is necessary.

- Jun 2nd 2006, 12:42 AM #2

- Joined
- Jan 2006
- From
- Brussels, Belgium
- Posts
- 405
- Thanks
- 3

a) On [0,1], f(x) is non-negative so the integral can be at least 0. But we know that f(1/2) > 0. So because of continuity, there exists an e > 0 for which f(x) > 0 on the interval (1/2-e,1/2+e). Then:

Now, the first and third integrals may be 0, but the second one is > 0.

b) If f isn't continuous, then you can construct a function for which f(x) > 0 for x in (1/2-e,1/2+e) isn't true. For example; keep f(x) at 0 everywhere, except at x = 1/2.