# First Derivative Test problem

1. Nov 2, 2005

### physics_ash82

Hi I need help using the First derivative test on this problem: f(x)= sinx divided by 1 + cos^2x . any help would be awesome.

2. Nov 2, 2005

### Pseudo Statistic

What's the first derivative test?
I've heard of the second derivative test... but never the first..
Do you mean finding the minimum/maximum?

3. Nov 2, 2005

### physics_ash82

yes That is what I eventually need, I can usually get the answer after I find f '(x) but the 1+ cos^2x is the part I cant figure out can you help?

4. Nov 2, 2005

### Pseudo Statistic

OK,
First, this should be in the homework section, but nobody likes a slut, so just disregard this sentence. :D
f(x) = [sin x]/(1 + cos^2 x)
Differentiate using the quotient rule and the chain rule.. (for cos^2 x, which would mean (cos x)^2 which would give you the derivative 2(-sin x)(cos x) = -2sin x cos x)
Thus, you have:
f'(x) = (cos x)(1 + cos^2 x) + 2 sin^2 x cos x all over (1 + cos^2 x)^2...
Set that equal to 0 and ignore the denominator for the moment... and see what solutions you get...
Chances are one of them might just be when 1 + cos^2 x = 0 (I don't know anything about trig functions more than the basics without a calculator, so don't approach me and prove me wrong, I'm trying to help :P) and.... yeah.... I guess that'll be an asymptote...
Hope this helped some.

5. Nov 2, 2005

### physics_ash82

Thank you for your help that made more since than what I got from class :P

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