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: Limit question

  1. Oct 13, 2006 #1
    Find the coordinates of the point on the curve f(x)=3x^2-4x, where the tangent is parallel to the line y=8x.

    Ok i know the slope of the tangent is 8.

    and i know the formula is m=[f(a+h)-f(a)]/h

    i need a hint please
  2. jcsd
  3. Oct 13, 2006 #2
    What is the derivative of [tex] f(x) [/tex]? Use the power rule, set the derivative equal to 8, and solve for x. Plug x back into f(x), and you got the point.
    Last edited: Oct 13, 2006
  4. Oct 13, 2006 #3
    umm we havnt been taught derivative's yet. This is an introductory to it.
  5. Oct 13, 2006 #4
    your title is "limit question." have you been taught:

    [tex] f'(x) = \lim_{h\rightarrow 0} \frac{f(x+h)-f(x)}{h} [/tex]? Use this definition for [tex] f(x) = 3x^{2} - 4x [/tex].
  6. Oct 13, 2006 #5
    yes i have been taught that

    but don't you need an x or y value of the tangent to use that formula.
  7. Oct 13, 2006 #6
    You know that the slope of the curve at the unknown point is 8 (because it is parallel to y = 8x). You don't need any values to use that formula. Just substitute (x+h) into f(x), and expand it out. After doing this subtract 3x^2 - 4x from f(x+h). Divide by h. Then substitute h = 0 to get f'(x).
  8. Oct 13, 2006 #7
    question solved
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook