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: Ax^2-40x+40=0 homework problem help

  1. Jul 12, 2004 #1


    User Avatar

    if you can solve ax^2-40x+40=0 for a and x that would be great
  2. jcsd
  3. Jul 12, 2004 #2


    User Avatar
    Science Advisor
    Homework Helper

    No. We have two unknowns (a and x) and one equation. At best, we can express a in terms of x, and vice versa, but no better. To express x in terms of a, simply apply the quadratic formula to the equation you have. You'll get:

    [tex]x = \frac{20 \pm 2\sqrt{10(10 - a)}}{a}[/tex]
  4. Jul 12, 2004 #3
    use this formula [tex]\frac{-b ^+_- \sqrt{b^2 - 4ac}}{2a}[/tex]where a=a, b=-40 and c=40

    Paden Roder
  5. Jul 12, 2004 #4
    Well, sorry! I just stated the quadratic formula, as AKG said. This was the first time I have used LaTeX though, pretty cool, once you get the hang of it.

    Paden Roder
  6. Jul 12, 2004 #5


    User Avatar

    i still don't get it can some one give me a walk through on how to use them with this problem/
  7. Jul 12, 2004 #6


    User Avatar

    a= 0 x= 1
    i just fount it
  8. Jul 12, 2004 #7

    I bet there are millions of answers for this one.

    I came up with a = 10, x = 2
  9. Jul 12, 2004 #8

    I bet there's more than that. In fact, I bet the set of all possible answers is uncountably infinite. I'd bet a lot of money on it, since, as AKG said, with one equation and two unknowns, this isn't going to go anywhere.
  10. Jul 12, 2004 #9


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    Now, while the others have shown how to find x given a, here's how you may find the a-value for any choice of x-value:
    Rewrite this as:

    Hence, for any non-zero choice of x, only a single value for a is allowed by the equation.
  11. Jul 24, 2004 #10
    Is this the whole question?
    Unless it's an 'express x in terms of a' type question you can't solve it with only that information.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook