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Homework Help: Differentiate lnsecx

  1. Jun 5, 2008 #1
    1. The problem statement, all variables and given/known data
    Given that y = lnsecx, - pi/2 <=x<=0, use the substituation u = secx, or otherwise, to show that dy/dx = tan x.

    3. The attempt at a solution

    well i thought about using the product rule, but you as it's ln(secx) not lnxsecx (2 different functions).... soooo im all out of ideas :(

    Thanks :)
  2. jcsd
  3. Jun 5, 2008 #2
    You could probably use chain rule...as in:

    << complete solution deleted by berkeman >>

    Don't quote me on this, I'm still learning basic calculus.=D
    Last edited by a moderator: Jun 5, 2008
  4. Jun 5, 2008 #3


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    FYI, it's preferable to just give hints, like the part I quoted above, rather than give complete solutions as you did.


  5. Jun 5, 2008 #4


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    Hi thomas49th! :smile:

    The question says:
    So try it … u = secx, so du = … ?, and y = … ? :smile:
  6. Jun 5, 2008 #5
    chain rule:

    dy/dx = dy/du . du/dx

    dy/du of lnsecx = 1/secx

    du/dx = secxtanx

    1/secx . secxtanx = tanx

    Cheers :)
  7. Jun 6, 2008 #6
    Whoops sry guys, I'm still relatively new to the forum. My apologies.
  8. Jun 6, 2008 #7


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    urrh …I was asleep! :zzz:

    Listen for the snoring next time!
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