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 to find the root of a quadratic formula

  1. Aug 10, 2011 #1
    1. The problem statement, all variables and given/known data

    (A) For what value of K does the equation 2x^2-10x +8 = K have a repeated root? Use the quadratic formula to find the root.

    (B) Choose a value for K so the equation has no real roots, Use the quadratic formula to prove that this equation has no real roots for your chosen value for K.

    this is a question i was given and to be honest i am stumped, i understand what a root is, but im unsure of how to find a repeated root. any help is appreciated!
  2. jcsd
  3. Aug 10, 2011 #2
    Do you know the quadratic formula? What would have to be the value of b2-4ac to have a repeated root?
  4. Aug 10, 2011 #3
    the formula above is the quadratic formula for both (A) and (B)

  5. Aug 10, 2011 #4
    No, the quadratic formula is a way to solve (i.e., find the solutions of) ax2+bx+c = 0
  6. Aug 10, 2011 #5
    this is the whole question i was given.

    2 a) draw the graph of the quadratic y=2x^2-10x+8

    2 b) solve the equation 2x^2-10x+8=K when K=0

    2 c) For what value of K does the equation 2x^2-10x +8 = K have a repeated root? Use the quadratic formula to find the root.

    2 d) Choose a value for K so the equation 2x^2-10x +8 = K has no real roots, Use the quadratic formula to prove that this equation has no real roots for your chosen value for K.

    does that help at all?
  7. Aug 10, 2011 #6


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    druuuuuuuunnk, you will have to change to soooooooober so that you can think about solving this problem.
  8. Aug 10, 2011 #7


    User Avatar
    Homework Helper

    SO have you done part (a) - it would be useful to factorise the quadratic in order to do that.
  9. Aug 10, 2011 #8
    Ok, I see where the confusion is. You might want to look at the part of the link that is called http://en.wikipedia.org/wiki/Quadratic_equation" [Broken].

    Then, the question is asking, for what value of K does the quadratic formula give repeated roots?
    Last edited by a moderator: May 5, 2017
  10. Aug 10, 2011 #9


    User Avatar
    Science Advisor

    Check in your textbook for the difference between "a quadratic equation" and "the quadratic formula".
  11. Aug 11, 2011 #10


    User Avatar

    Staff: Mentor

    A quadratic can have two roots, or no roots. (Or to put it more exactly: two real roots, or no real roots.)

    If the real roots are equal, then this can be referred to has having a repeated root. So all you need to do is determine that value of K which causes there to be zero difference between the two roots.
    Last edited: Aug 11, 2011
  12. Aug 11, 2011 #11
    ok so I've been doing some reading, i now know that i use the quadratic forumla to solve the quadratic equation in part B. i got x=4 and x=1

    for part C, do i change the quadratic equation 2x^2-10x+8=K. by this i mean do i need to make it 3x^2-11x+9=K or somthing like that, until i find a equation that has two roots which are the same?

    im not gonna lie to you guys, once i've done this maths course work i pass my year and i never have to do maths at this level again. Im doing product design at university. its basically just drawing lol.

    Im terrible at maths, my uni are making me do a foundation year or maths and physics, they are not my strong points, if you could just tell me the answer i would appriate it, this course works in for tomorrow and its worth 50% of my final grade. if i get 80% of this course work right i pass the year.
  13. Aug 11, 2011 #12


    User Avatar
    Homework Helper

    No. You move the K to the other side, and let c = 8 - K in the quadratic formula. You'll notice that in order for you to have a repeated root, the expression underneath the square root, b2 - 4ac, must equal 0. Look up discriminant.

    No, we're not allowed to just give the answers. You need to show the work to us first, and then we help you. That's the forum rules.
  14. Aug 11, 2011 #13


    Staff: Mentor

    From the forum rules (https://www.physicsforums.com/showthread.php?t=414380):

    Homework Help
    On posting questions: Any and all high school and undergraduate homework assignments or textbook style exercises for which you are seeking assistance are to be posted in the appropriate forum in our Homework & Coursework Questions area--not in blogs, visitor messages, PMs, or the main technical forums. This should be done whether the problem is part of one's assigned coursework or just independent study. The reason for this is that the scientific and mathematical sections of Physics Forums are to be reserved for discussions and not academic assistance. Since graduate level assignments are meant to be more thought provoking (and hence more worthy of discussion), graduate level questions will be allowed in the relevant part of the main section of PF, provided that the graduate student attempts the problem and shows his work.

    NOTE: You MUST show that you have attempted to answer your question in order to receive help. You MUST make use of the homework template, which automatically appears when a new topic is created in the homework help forums. Once your question or problem has been responded to, do not go back and delete (or edit) your original post.

    We do not support cheating in any form: Do not ask for solution manuals, answers to exams, or instructor's manuals. Every school and instructor has their own policies or honor codes on what constitutes cheating, and it is up to the individual student to adhere to those policies when seeking help here. If you are in doubt as to whether you are permitted to seek help, consider erring on the side of caution and not asking for help.

    On helping with questions: Any and all assistance given to homework assignments or textbook style exercises should be given only after the questioner has shown some effort in solving the problem. If no attempt is made then the questioner should be asked to provide one before any assistance is given. Under no circumstances should complete solutions be provided to a questioner, whether or not an attempt has been made.
  15. Aug 12, 2011 #14
    it was worth a try :P
  16. Aug 12, 2011 #15


    User Avatar
    Science Advisor

    Well, it should have occured to you that people who frequent mathematics sites are probably not going to help someone avoid learning mathematics- and that is what you are saying you want to do.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook