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: Solve this quadratic =(?

  1. May 3, 2008 #1
    first part of the question is simple :
    x^2 - 8x + 11 = 0

    solve. using quadratic formula it is 4 +/- root5

    second part confuses me, you are given to equation:

    y - 8y^(1/2) + 11 = 0

    and are told to :

    solve this giving answer in form p +/- Q * root5

    i have no idea how to do this. the quadratic formula doesnt work on this one and i dont understand how part 1 of this question helps me with this part, can someone explain it to me please.!? thanks
  2. jcsd
  3. May 3, 2008 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    Hint: Can you express y in terms of x, such that your second equation becomes your first?
  4. May 3, 2008 #3
    i don't know how to do that, i've tryed making the equations equal to each other but it doesn't work out right
  5. May 3, 2008 #4


    User Avatar

    Staff: Mentor

    Compare the equations term by term.


    next is...
  6. May 3, 2008 #5
    oh yeah, i can do that because they're both 0, and are of the same form. lewl
  7. May 3, 2008 #6


    User Avatar
    Science Advisor
    Homework Helper

    Hint: substitution. :smile:
  8. May 3, 2008 #7
    i've honastly tried for ages, i still dont know how to do it.

    can someone just run me through it. it's not even a question for homework or anything im just revising and i dont understand this.
  9. May 3, 2008 #8


    User Avatar
    Science Advisor
    Homework Helper

    Bigger hint: substitute y = x². :smile:
  10. May 3, 2008 #9

    why can i just substitute that.
  11. May 4, 2008 #10


    User Avatar
    Science Advisor
    Homework Helper

    eh? :confused: You can substitute anything you like. :smile:

    Some substitutions make the problem easier :!!) , some substitutions make it harder. :mad:

    But all substitutions are valid.

    Try it … put y = x² into y - 8√y + 11 = 0, and see what happens! :smile:
  12. May 4, 2008 #11
    think about it a second:
    you've already found that [tex]x^{2} - 8x + 11[/tex]
    Now you need to find [tex]y - 8y^{1/2} + 11[/tex]
    If you substituted [tex]x^{2}[/tex] = y,
    you would have [tex]x^{2} - 8x + 11[/tex]
    which you already have the answer to. If x = 4+/-[tex]\sqrt{5}[/tex],
    what is [tex]x^{2}[/tex] (i.e. y) going to equal?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook