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: Function and its tangent

  1. Jan 26, 2010 #1
    1. The problem statement, all variables and given/known data

    Find the equation f(x) who's integral is [tex]\int x^{3}[/tex] and has a tangent x+y=0

    2. Relevant equations

    3. The attempt at a solution

    I know that f(x) is 1/4x4+c because of the integral. The tangent is the derivative of f(x) at some point

    i have the equations


    but solving these equations gives me two unknowns...
  2. jcsd
  3. Jan 26, 2010 #2

    Char. Limit

    User Avatar
    Gold Member

    Well, at what point does x^3 equal negative 1 (the slope of y=-x)
  4. Jan 26, 2010 #3


    Staff: Mentor

    What you have written is not very clear. From what I can tell, f(x) = (1/4)x4 + C.

    You're given that f is tangent to the graph y = -x. This means that f'(x) = x3 has to equal -1 (the slope of the line y = -x). It also means that the graph of f has to have a point in common with the line y = -x.
  5. Jan 26, 2010 #4


    f(x)= 1/4x4+3/4
  6. Jan 26, 2010 #5


    Staff: Mentor

    -1 for what?
    Why 1? You're not explaining what you're doing, which makes it extremely difficult to understand your work.
  7. Jan 26, 2010 #6
    Sorry, I was replying to Char. limit's question
  8. Jan 27, 2010 #7


    Staff: Mentor

    Can you post the problem exactly as it is worded? I'm having a hard time believing this is what you have actually been given:
    especially the part that says "who's integral is [itex]\int x^{3}[/itex]..."
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook