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

Deriv of xtanx

  1. Oct 9, 2003 #1
    y = x tan x

    y' = tan x + x*sec^2(x)

    y'' = sec^2(x) + d/dx (x*sec^2(x))

    = ... + (d/dx(sec^2(x)) + sec^2(x))

    = ... + (2secx + sec^2(x))

    = 2sec^3 x + sec^4(x)

    what did I do wrong?? in book it says y'' = something else.
     
  2. jcsd
  3. Oct 9, 2003 #2

    mathman

    User Avatar
    Science Advisor
    Gold Member

    First term should have d/dx(secx)=tanx.secx multipled, i.e. first term is 2sec2x.tanx.
     
  4. Oct 10, 2003 #3

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    You multiplied the first "sec^2 x" instead of adding!
     
  5. Oct 10, 2003 #4
    I found the answer this morning,

    something like

    2sec^2 x(tanx + 1)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?