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Homework Help: Finding Infliction points and the equations of their slope

  1. Sep 16, 2011 #1
    1. The problem statement, all variables and given/known data
    x^4 - 6x^3 + 12x^2 - 8x

    a) Find all points of infliction of this function
    b) Sketch the function and identify all convex and concave portions of the curve
    c) Find the equations of the slopes at each point of infliction

    2. Relevant equations
    x^4 - 6x^3 + 12x^2 - 8x

    3. The attempt at a solution
    What I attempted to do was to do the second derivative of the function (as I dont need to determine min/max there is no need to bother with the first derivative right, i.e. finding the points at which the slope is 0?). After finding the second derivative I just solve it such as that y''=0 for a given x.

    y' = 4x^3 - 18x^2 + 24x - 8
    y'' = 12x^2 - 36x + 24

    Solving X such as that y'' = 0 (infliction points):
    [-(-36) +/- sqrt(-36^2 - (4*12*24)] / 2*12

    Which yields: (36+/- 12) / 24 ----> x = 2, x = 1

    So the infliction points are x=2 and x=1.

    I think I got this one right as it's just a matter of plugging in different values for X and plot it in a graph.

    This is the one I cant figure out how to do. I am supposed to find the equations of the slopes at each point of infliction, but I dont quite understand the question. Do I just plug in the x value in the original function?
    Ex. x = 1 ---> Equation at infliction pt: 1-6+12-8 = -1 ??

    I would really appriciate some guidance as to how I am supposed to do c).

  2. jcsd
  3. Sep 16, 2011 #2
    Take a look here: http://en.wikipedia.org/wiki/Slope#Calculus"

    The slope of a non-linear function is given by the derivative of the function in a certain point, so plug the x-values into the y' you found :smile:

    Hope you're helped by this !

    You were asked to find the equation of the slope, therefor you might as well take a look here:http://en.wikipedia.org/wiki/Tangent#More_rigorous_description"

    The last link describes how to find the equation when you know the x and y values of the point, and the slope of the function's tangent in the point.

    Then you will 've reached your goal... I guess
    Last edited by a moderator: Apr 26, 2017
  4. Sep 16, 2011 #3
    Thanks a bunch mate! That definately helped!
    But how is that "an equation" as y' will equal 0 at x=2 and equal 2 at x=1? Isn't that just numbers ;)

    I thought you had to find some type of equation in the sense (x + y....). English is not my first language so sometimes when I'm doing maths in english I get confused :)

    Again, thank you very much!
    Last edited by a moderator: May 5, 2017
  5. Sep 16, 2011 #4
    You should find an equation, did you see I edited my first post, because what I first thought was that you should find the slope, and not the equation of the slope... Sorry , I should rather have made a new post

    To find the equation when having x-value in the point, y-value in the point (found by plugging into y), and the "value" of the slope y'(x), you use:

    y="y-value"+ "slope value" times (x-"x-value")

    Again, sorry for the inconvenience....

    It took me a second more to understand your question fully than to press the "submit reply" button... since English is not my first language either :)
  6. Sep 16, 2011 #5
    hey mstud - no worries at all! Now I learnt 2 new things even though I just asked for 1 ;)

    Ok, after reading your post and the wikipedia page i tried to solve it:

    x = 2 (infliction point found earlier)

    y = f(2) + f'(2)(x-2)

    --f(2) = 2^4 -6(2^3) + 12(2^2) - (8*2) = 0
    --f'(2) = 4(2^3) - 18(2^2) + (24*2) - 8 = 4

    Plugging that in the formula yields:

    y = 0 + 4(x-2) ----> y = 4x - 8

    Solving for x yields x = (y-8)/4

    Is this the right way of doing it or have I got it wrong once again :)
  7. Sep 19, 2011 #6
    The answer is y = 4x - 8 , just like you found. It is not necessery to solve for x, but if you should have done so, it would have been:

    [tex]y=4x-8 \Leftrightarrow y+8=4x \Leftrightarrow x= (y+8)/4[/tex]

    Nice work as far as you had to go, and I just wanted to comment your solving for x so you would know till next time :)
  8. Sep 19, 2011 #7
    oh yeah, that was a silly mistake by me, should have seen it's +8 and not - hehe.

    Thank you very much for your time and effort to help me out mate, I really appriciate it!
  9. Sep 19, 2011 #8
    Such mistakes we all make, sometimes.

    You're welcome!
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