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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)

    y=2x^2-10x+8
     
  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

    SteamKing

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

    PeterO

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    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

    HallsofIvy

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

    NascentOxygen

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    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

    eumyang

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    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

    Mark44

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

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  15. Aug 12, 2011 #14
    it was worth a try :P
     
  16. Aug 12, 2011 #15

    HallsofIvy

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    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.
     
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