# Relationships between first and second derivatives

1. Nov 25, 2008

### madgab89

1. The problem statement, all variables and given/known data
Figure 4.46 shows the second derivative of h(x) for -2 $$\leq$$ x $$\leq$$ 1

a) Explain why h'(x) is never negative on this interval.
b) Explain why h(x) has a global maximum at x=1
c) Sketch a possible graph of h(x) for this interval.

I realize this is probably a fairly simple question, however it's just making my head hurt.

http://i414.photobucket.com/albums/pp222/madgab89/Picture2.jpg

2. Nov 25, 2008

### Staff: Mentor

Are you sure you have given us all of the data? Perhaps the value of h'(x) at x=-1?

3. Nov 25, 2008

### madgab89

sorry, don't know how i could have missed that

h'(-1)=0
h(-1)=2

4. Nov 25, 2008

### Staff: Mentor

Much better.

The relation between the first and second derivative is that the second derivative of some function is the first derivative of the first derivative of that function.

So, to avoid this confusion, for a while let's denote h'(x) as g(x). Then g'(x)=h''(x). Now, if I showed you that graph in the first plot and labeled it as g'(x) rather than h''(x) and asked you to explain why g(x) is never negative on the interval in question, could you do that?

5. Nov 26, 2008

### HallsofIvy

Staff Emeritus
For $x\ge -1$, the graph shows that h"(x) is positive. That means h'(x) is increasing. And since h'(-1)= 0 ....

Now that you know that h' is positive on the interval, h(x) is increasing so its minimum and maximum on [-1, 1] must be at ...

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?