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: Very simple derivative

  1. Nov 6, 2008 #1
    1. The problem statement, all variables and given/known data
    3/x^2

    when I take the quotient rule ,
    I get:
    (0*x^2-3*2x)/(x^2)^2
    isn't that -6x/x^4 or -6/x^3
    My calculator says -6 and so it is, but why and what am I missing?
     
  2. jcsd
  3. Nov 6, 2008 #2
    ok From what i see you are right.the answer is -6x^-3.

    but at what point are you evalusting.did they give you a point or did you just type on your calculator.you have to have value of x.to type on your calculator
     
  4. Nov 6, 2008 #3
    I originally had to differentiate f(t)=cos^-1(3/t^2)
    my answer was f'(t)=-1/sqrt(1-(3/t^2)^2)*-6t
    f'(t)=6t/sqrt(1-9/t^4)
    I got the "-6t" part wrong, it was supposed to be just -6.

    when I differentiate just 3/x^2 into my calculator and it comes out -6
    on my ti89 calc. I go to math, calculus, differentiate and type in:
    d(3/x^2,x) and get -6
     
  5. Nov 6, 2008 #4
    Instead of using the quotient rule, you might find it easier to evaluate
    3 x^-2
     
  6. Nov 6, 2008 #5
    Oky, thanks. I see my problem.
    I should have looked into the mirror sooner, the problem was there all along!
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook