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: Proving using calculus without trig identity

  1. Feb 23, 2012 #1
    Please I really need help with this hw question

    Prove without trig identity that f`(x)=0 for

  2. jcsd
  3. Feb 23, 2012 #2
    You're not supposed to use the obvious identity that simplifies this? I suppose you could just use the derivatives of sin and cos along with the chain rule to directly compute the derivative. But eventually you'll need to simplify using some trig identity.
  4. Feb 23, 2012 #3
    I can't use trig identy to solve it
  5. Feb 23, 2012 #4
    I mean I'm not allowed to
  6. Feb 23, 2012 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    What is F'(x) if [itex]F(x)=A\sin^2(Bx+C)+A\cos^2(Bx+C)\,?[/itex]
  7. Feb 23, 2012 #6


    User Avatar
    Homework Helper

    Are you sure? I was able to get F'(x) = 0 by using the chain rule, and yet I didn't use any trig identity.
  8. Feb 26, 2012 #7
    could you please send me your working for this question??
  9. Feb 26, 2012 #8
    It's really simple - just use chain rule to take the d/dx of the whole expression. No trig or any other kinds of tricks necessary. Are you familiar with the use of chain rule?
  10. Feb 26, 2012 #9


    Staff: Mentor

    This is not permitted at Physics Forums - don't even ask.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook