Results 1 to 2 of 2

Math Help - df/dx when x=pi/4

  1. #1
    Member
    Joined
    Sep 2009
    Posts
    124

    Post df/dx when x=pi/4

    Let f(x)= (1+cos^3(7x))^3. what is df/dx when x=pi/4

    so far I have
    df/dx= 3(1+cos(7x)^3)^3 (3cos(7x)^2 (-sin(7x) (7)

    and when i plug in pi/4 i get 20.404 (when accurate to one-thousandths)

    is this right?
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Super Member Matt Westwood's Avatar
    Joined
    Jul 2008
    From
    Reading, UK
    Posts
    732
    Not sure df/dx is right. Think the first power should be a square not a cube (I may be wrong)

    And plugging a number in via a calculator to get a numerical answer is not mathematics. What you need to do is remember that cos (\pi / 4) = \frac {\sqrt 2} 2 and same for sine and use that.
    Follow Math Help Forum on Facebook and Google+

Search Tags


/mathhelpforum @mathhelpforum