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: How to find local max and min points for y = sinxcox^3x

  1. Apr 22, 2010 #1
    1. The problem statement, all variables and given/known data

    Hey, so i need some help trying to find the local max and min points for y = sinxcox3x

    2. Relevant equations

    3. The attempt at a solution
    I know i need to find the first and 2nd derivative but i do not know if i am doing it right. I also do not know what to do after wards.

    my first derivative ends up being cos^2x(cos^2x - 3sin^2x)
    what do i do after this to solve my question. Help would be appreciated.
    Last edited: Apr 22, 2010
  2. jcsd
  3. Apr 22, 2010 #2
    You need to find the first derivative and check the values of x when Dy = 0 - so set your derivative to 0 and solve for x.
    I did not check your derivative. Did you apply the product rule?
  4. Apr 22, 2010 #3
    I got the same derivative.
  5. Apr 22, 2010 #4
    Yes my derivative is cos^2x(cos^2x - 3sin^2x) so do i do

    0 = cos^2x or 0 =cos^2x - 3sin^2x)

    and solve for both?
  6. Apr 22, 2010 #5
    Yup you solve both
  7. Apr 22, 2010 #6
    Ok so i got the values for the left but the right eqn is nto going well. I do not understand what to do after cos^2x - 3(1-cos^2x)
  8. Apr 23, 2010 #7


    User Avatar
    Science Advisor

    [itex]cos^2 x- 3 sin^2 x[/itex][itex]= cos^2 x- 3(1- cos^2 x)[/itex][itex]= cos^2 x- 3+ 3cos^2 x= 0[/itex]
    so [itex]4cos^2 x= 3[/itex], [itex]cos^2(x)= 3/4[/itex].
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook