# Homework Help: Intervals of increase and decrease

1. Oct 30, 2012

### ifi2world

1. The problem statement, all variables and given/known data
find the intervals of increase or decrease of the function f(x)= (3/(x^2+11)-1

i already find the 1st derivative f'(x)= -6x/ (x^2 +11). After that i didnt know how to proceed to find the interval. I need help for the solutions.

Last edited: Oct 30, 2012
2. Oct 30, 2012

### Sourabh N

How does the first derivative behave in the interval of increase?

3. Oct 30, 2012

### Saitama

Do you mean
$$f(x)=\frac{3}{x^2+11}-1$$
If so, check the f'(x) you have calculated. The denominator should be squared.

4. Oct 30, 2012

### ifi2world

yes.. sorry for typo error.
so what should i do next to get the interval?
i need to find x after i did the 1st derivative but how to square root the -11? isnt that impossible or does it have another formula?

5. Oct 30, 2012

### Saitama

If you examine the derivative, the denominator is squared and is always positive, so we don't really need to worry about that. Now see the numerator, what is the sign of the expression when x<0, what it is when x>0?
Not really following what you are asking here.

6. Oct 30, 2012

### ifi2world

the sign is + for x<0 & - for x>0...so next is i need to find the domain for the function for the interval value...right?