Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: First Derivative Test problem

  1. Nov 2, 2005 #1
    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.:confused:
  2. jcsd
  3. Nov 2, 2005 #2
    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?
  4. Nov 2, 2005 #3
    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?
  5. Nov 2, 2005 #4
    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.
  6. Nov 2, 2005 #5
    Thank you for your help that made more since than what I got from class :P
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook