Sketch the graph of an arbitrary continuous function from the given information. Based on this, describe the local extrema of the function.

calculusderivatives

Suppose that $f$ is a continuous function on $[0,10]$. Sketch the graph from the following description:

  1. $f$ is 0 at 4, 7 and 9
  2. $f'(x)>0$ on $0<x<5$ and $8<x<10$
  3. $f'(x)<0$ on $5<x<8$

With this information, can you say anything for certain about the maximum/minimum value of $f$? Can you say anything about the values of $x$ for which an extremum is obtained?

This problem(2B-4) appears in MIT's 18.01 Exercise Set 2 on Applications of Differentiation. The graph(provided as a solution) is as shown below:
graph

I have some doubts pertaining to this solution:

  1. The differentiability of function is not clear. Although the increasing/decreasing values of derivatives are provided, the function was only described as continuous on $[0,10]$. Further, the behaviour of the derivative is not defined for 0, 5, 8 and 10.
  2. In the absence of knowledge about derivative at $x=8$, Rolle's Mean Value Theorem can't be used to claim the existence of a stationary point in the interval (7,9).
  3. Even if the derivatives were all well-defined, i.e., the function was differentiable over (0,10), are the claims about the maximum occurring at $x=5$, or the minimum occurring at $x=8$ legitimate?

Best Answer

Ironically as I finished typing my question, the solution struck my mind: that $x=5$ and $x=8$ are turning points of $f$, regardless of the differentiability of $f$ at these points. Since $f'(x)>0$ for $x<5$, and $f'(x)<0$ for $x>5$, $f$, being continuous, has a local maximum at $x=5$(similar reasoning follows for $x=8$).

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