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: How can I find the max/min of this function?

  1. Jun 3, 2013 #1
    1. The problem statement, all variables and given/known data
    Find the critical point of f(x)=(-5x^2+3x)/(2x^2-5), and show whether this point is a max/min.

    2. Relevant equations


    3. The attempt at a solution
    When I tried solving for the derivative of; f(x)=(-5x^2+3x)/(2x^2-5), I got (-6x^2+50x-15)/2x^2-5)^2.

    then I set it equal to 0 and I got 0=-6x^2+50x-15

    Now from here do I use the quadratic formula to solve for x? I tried that and I am getting x=(25+sqrt535)/6 and x=(25-sqrt535)/6. Have I done it right so far? When I try to plug in the x-values into the original eq to get y my calc says error... What do I do now?
  2. jcsd
  3. Jun 3, 2013 #2
    Try again. Everything looks OK.
  4. Jun 3, 2013 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Make sure you are using the right quadratic formula. Remember, it starts out with '-b', and your answers don't have the correct value for '-b'.
  5. Jun 3, 2013 #4


    User Avatar
    Education Advisor

    The op's quadratic answer is correct.* If you're plugging in the exact formula into the calculator, I would simply instead just take an approximation of your answers (ie .311 and 8.02) and plug that in and then proceed with determining if the values are a max or a min.

    *(The -b is negated by the 2a, where a is negative.)
    Last edited: Jun 3, 2013
  6. Jun 3, 2013 #5

    Ray Vickson

    User Avatar
    Science Advisor
    Homework Helper

    Your answers are correct; it sounds like you need to buy a new calculator.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted