1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: I am having trouble finding trig derivatives using chain rule

  1. Sep 27, 2011 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations
    chain rule
    f prime (x) = f prime(g(x) * g prime (x)

    3. The attempt at a solution
    I am not sure if I am just inputting the wrong numbers into webassign or I am just missing and important trig derivative and just completely off of the boat here.

    the answer I got the first time is:
    -2 cot(cos[itex]\theta[/itex])*[-csc^2 (cos[itex]\theta[/itex]) *-sin[itex]\theta[/itex])

    thanks for any help you guys can give me
  2. jcsd
  3. Sep 27, 2011 #2
    Pretty close, you have an extra minus sign, that's all. How'd you arrive at that negative out in front?
  4. Sep 27, 2011 #3
    It looks like using the example I was looking at I distributed one extra negative sign
  5. Sep 27, 2011 #4
    I think there is one more step that I am lost on. I ended up getting 2 cos theta *cot(cos theta) *csc^2(-sin theta), which webassign did not like :( ahhhh
  6. Sep 27, 2011 #5
    You had the answer above already, with the exception of the extra minus sign.

    To clarify, the question is:

    What is the derivative of [itex]\cot^2(\cos \theta)[/itex]?

    You have already answered it above... how did you arrive at the second post?
  7. Sep 27, 2011 #6
    yes, the question you have is right.
    I worked the problem by looking at an example video that comes with our online book.
    the instructor in the video further broke down from the answer in the sample problem, which was cot^2(sin theta), which the answer is -2cos theta * cot(sin theta) *csc^2 (sin theta)
  8. Sep 27, 2011 #7
    oh i see what I did wrong!!!! apparently the homework program did not like that I did not put a space between [-csc^2 and cos theta) wow here I was thinking I did not know what I was doing and it was just an input error, not a mathematical error! Got to love technology sometimes!
  9. Sep 27, 2011 #8
    Glad to hear you got it.
  10. Sep 27, 2011 #9
    thanks for the quick replies! sometimes it just takes a "fresh set of eyes" to say hey what are you doing there!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook