1. Not finding help here? Sign up for a free 30min 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!

Calc Question

  1. Mar 1, 2007 #1
    1. The problem statement, all variables and given/known data

    Find a quadratic function f(x), that satisfies the given conditions: f(1) = 2,
    f(-1) = 4, f(3) = 8.


    2. Relevant equations

    f(x) = k(x-s)(x-t)

    3. The attempt at a solution

    I tried entering the points as the the x and f(x) while estimating as what the zeroes would be and seeing if k values would be the same but it did not work.

    I am pretty sure that the first zero is 2 < x < 4, and the second zero is 4 < x < 8. But I have no idea where to go now. Any help would be greatly appreciated.
    Last edited: Mar 1, 2007
  2. jcsd
  3. Mar 1, 2007 #2

    D H

    Staff: Mentor

    This is not a good form to use when solving this kind of problem. You get terms like [itex]kst[/itex], which are very hard to deal with. Try using a different form, for example [itex]f(x) = ax^2 +bx + c[/itex].
    Last edited: Mar 1, 2007
  4. Mar 1, 2007 #3
    how do you find a, b and c?
  5. Mar 1, 2007 #4

    D H

    Staff: Mentor

    Don't use calculus. With that form, what are f(-1), f(1), and f(3)?
  6. Mar 1, 2007 #5
    Is it by trial and error or is there a simpler way to do it?
  7. Mar 1, 2007 #6


    User Avatar
    Staff Emeritus
    Science Advisor

    Just put in each value of x, and you will obtain 3 equations which you can solve for a, b and c.
  8. Mar 1, 2007 #7

    D H

    Staff: Mentor

    For example, [itex]f(1) = a*1^2 + b*1 + c = a+b+c[/itex]
  9. Mar 1, 2007 #8


    User Avatar
    Staff Emeritus
    Science Advisor

    This is hardly a "Calc" problem- more like basic algebra

    [itex]f(1) = a*1^2 + b*1 + c = a+b+c= 2[/itex]

    Do the same with the other two values you are given so you have three equations for a, b, and c. Solve the equations.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Calc Question
  1. Calc Question (Replies: 3)

  2. Calc question (Replies: 6)

  3. Calc question (Replies: 2)

  4. Questions on calc 1 (Replies: 12)