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: Lost learning derivatives

  1. Feb 22, 2006 #1
    Hello, today my teacher presented us with the concept of derivatives. I was okay in class, but when I got home, I was completly lost. Using the definition (f(x+h) - f(x))/h, we have to fine the derivative of y=2x^2 - 5. I know the answer is 4x from the back of the book, but I cant understand how to get it. Am I factoring something wrong? I get to a point of simplification that everything just cancels out. Your help is greatly appriciated!
  2. jcsd
  3. Feb 22, 2006 #2
    The definition you want to use is this:

    [tex]\lim_{h\rightarrow 0}\frac{f(x+h) - f(x)}{h}[/tex]

    Now, set [itex]f(x)=2x^2-5[/itex] and insert this in the expression, simplify and take the limit.
  4. Feb 22, 2006 #3
    Quick check: When you expand the numerator, f(x+h)-f(x), you should get an expression that cancels the h in the denominator.

  5. Feb 22, 2006 #4
    my problem is when i simplify, i get 0 over 0....i dont know why....i cant get the h to go away in the denominator
  6. Feb 22, 2006 #5
    [tex] f(x+h)=2(x+h)^2-5=2x^2+4xh+2h^2-5 [/tex]
    [tex] f(x)=2x^2-5 [/tex]
    So when you subtract the two you get...

    Last edited: Feb 22, 2006
  7. Feb 22, 2006 #6
    Then you must be doing something wrong. Remember this: If [itex]f(x)=2x^2-5[/itex], what is then [itex]f(x+h)[/itex]? Check your calculations again.
  8. Feb 22, 2006 #7
    Thank you so much guys!!! Wow i feel dumb....turns out i was just plugging the functions in wrong. Thank you so much!!!!
  9. Feb 22, 2006 #8
    Hey, as I always tell my students: it's only a dumb question when you DON'T ask it! :biggrin:

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook