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Let $f$ be a real valued function continuous on the closed interval $[0,1]$ and differentiable on the open interval $(0,1)$, with $f(0)=1$ and $f(1)=0$. Which of the following must be true?

  1. There exists $x \in (0,1)$ such that $f(x)=x$.
  2. There exists $x \in (0,1)$ such that $f'(x)=-1$.
  3. $f(x)>0 $ for all $x \in [0,1)$

I think the solution is 1 only, but apparently is 1 and 2 only. I don't see why 2 is true, if the derivative of $f$ is not necessarily continuous.

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Wow. I was confused for a very long time. You're 2 is not on a new line, and then the one below it is labeled 2, so I was missing the real 2 and looking at 3 thinking that it is clearly false. I'll edit that I guess...OK, actually already done by MJD. –  Matt Oct 7 '12 at 23:19

4 Answers 4

up vote 8 down vote accepted

This is a direct application of the mean value theorem, which does not require $f'$ to be continuous.

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Let $g(x)=f(x)+x$. Note that $g(0)=g(1)=1$.

If $g(x)$ is identically equal to $1$, there is nothing to prove.

If $g(x)$ is not identically equal to $1$, then $g$ attains a local maximum or minimum in $(0,1)$. There we have $g'(x)=0$.

Remark: Even MVT was not used, though admittedly the crucial component of the proof of MVT was.

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The derivative may not be continuous, but it does have the Intermediate Value Property.

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How does this help? We aren't given values of $f'$. –  Potato Oct 7 '12 at 23:17
1  
Can you show it's impossible for $f'\gt-1$ for all $x$? ditto, $f'\lt-1$ for all $x$? –  Gerry Myerson Oct 7 '12 at 23:26

Direct application of mean value theorem.

It is differentiable in open interval $(0, 1)$. Hence it is continuous too. So no matter what the graph is, it will have some point where the slope would be $-1$.

From $x=0$ to $x=1$, you have to connect $(0, 1)$ (as f(0)=1) and $(1, 0)$ (as f(1)=0).

You can consider some cases to do so.

Case 1:

A curve first going up, then coming down. So while coming down, it will have -1 slope somewhere

Case 2

A curve first going down, then coming up. While going down, it will have -1 slope somewhere

Case 3

A straight line connect the two points. Clearly, it has a slope of -1

So, 2$^{nd}$ must be true. There can be infinite curves. I've explained just three.

While doing the question, if you can't figure it out, just try to search for a contradicting example.

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