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: Curve sketching

  1. Dec 21, 2012 #1
    1. The problem statement, all variables and given/known data

    y = 1/ (3+x^2))

    2. Relevant equations

    y int; y =1/3 (((0,1/3)
    lim x- (1/x^2) / (3/x^2+ x^2+x^2))
    y =0
    VA: none

    x_ infine x__ + infinite { next two lines are under it, for x infinite and x + infinite)

    TV -1000 1000
    y= 0 0

    above above

    f '(x) =[ (3+x^2)(0) - 1(2x) ] /[ ((3+x^2)^2 ]
    = 2x / (3+x^2)^2

    fx = 0 at x = 0

    what else am i missing?? to draw a complete graph : teacher said alot was missing and try again ,but i dont see anything missing!!!!
    3. The attempt at a solution
  2. jcsd
  3. Dec 21, 2012 #2


    User Avatar
    Homework Helper
    Education Advisor
    Gold Member

    y = 1/ (3+x^2))

    Before even using derivatives, see that because the denominator is always positive, y is always positive, above the x axis. x can be zero, rendering the largest possible value for y. As x becomes either more negative without bound or more positive without bound, the rational expression moves closer to zero but will never reach zero.

    You should find a maximum at x=0 and asymptote y=0.
    Derivatives may give you a bit other information, like inflection points if you find second derivative... you should study how. The first derivative should be equal to zero at either a maximum or a minimum; and in the example given, you should expect to find a maximum (at x=0).
  4. Dec 21, 2012 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    It helps us to help you, if you give a complete statement of the problem.

    Let's see ...

    You've given us a title, "curve sketching" , an equation giving y as a function of x, then below (at the very end), you mention that the work you have shown here "has a lot missing" according to your teacher..

    I guess you are supposed to use calculus and some algebra skills to determine all you can regarding the given function so that you can adequately sketch a graph of the function.

    It took me quite a while to figure out what you were trying to do in the above snippet.

    It looks like you're trying to find a bit more than simply the HA for x → ± ∞ , by using some fairly large test points.
    You have a sign error in your first derivative.
    [itex]\displaystyle f'(x)=\frac{-2x}{(3+x^2)^2}[/itex]​
    Where is f '(x) > 0 ? Where is f '(x) < 0 ?

    What does it mean that f '(0) = 0 ?

    What is f ''(x) ? Where is it +, 0, - ?

    Are f(x) f '(x), f ''(x) even or odd respectively?
  5. Dec 22, 2012 #4


    User Avatar
    Homework Helper
    Education Advisor
    Gold Member

    REALLY BIG HINT: The numerator of the simplified form of the second derivative is

    Where is f''(x)=0?

    Another hint, just as important: Where is f'(x)=0 and what does this mean?
  6. Dec 22, 2012 #5
    i guess i have done a lot wrong from the start!!!

    can you tell me exactly the steps i need to start from the beginning using calculus.

    all i know is to find y intercept, horizontal asymptote, V.A , using the test values and finding how the graph will look...

    what else!! please help
    Last edited by a moderator: Dec 22, 2012
  7. Dec 22, 2012 #6


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    It's not that you did all that much wrong. You had a sign error in the first derivative.

    Mainly, as your teacher said, you could have gone much further.

    You found the y-intercept, but should have mentioned that there's no x-intercept.

    Did you notice that your function is even?

    You found the Horizontal Asymptote and also noted that there's no Vertical Asymptote. At this point you might observe that the domain of the function is (-∞, +∞).

    Determining that the function approaches the HA from above was fine. It just took me some time to figure out what you were doing how you were doing it.

    The first derivative is [itex]\displaystyle \ y'=-\frac{2x}{(3+x^2)^2}\ .[/itex] You can use that to find the min/max for the function, and also to find where the function is increasing/decreasing as y' is positive/negative.

    Use the second derivative to find any inflection points and to determine the concavity of the function.

    Somewhere along the line, you could also determine the range of the function.
  8. Dec 23, 2012 #7
    thanks , that helped me get back on track.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook