# Homework Help: Caculus: Critical Numbers

1. Jan 24, 2008

### Hothot330

1. The problem statement, all variables and given/known data
The first derivative of the function f is given by f '(x)= (((cos^2)x)/(x)) - 1/5
How many critical numbers does f have on the open interval (0,10)?

2. Relevant equations

3. The attempt at a solution
I already got this question wrong but I don't know why I got it wrong. The answer is 3 but when I graph it I see 6 critical numbers. So why is it 3 and not 6? Please explain.

2. Jan 24, 2008

### rocomath

Let's clear up any confusion about what the problem is ...

$$f'(x)=\frac{\cos^2 x}{x}-\frac 1 5$$

Correct?

3. Jan 24, 2008

### Hothot330

Correct

4. Jan 25, 2008

### Hothot330

I tried taking the derivative of the derivative and graphed that but it still gives me an image of 6 critical points.

5. Jan 25, 2008

### HallsofIvy

I don't understand why that would happen. A critical point occurs where the derivative is 0 or does not exist. Clearly your derivative does not exist at x= 0. To determine where f'= 0, I graphed y= 5cos2(x) and y= x. They cross in 3 points.

Wait, did you differentiate again? You said what you gave was f '. To determine where f has critical points, you should be graphing that, not its derivative.

Last edited by a moderator: Jan 25, 2008