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!

B Why is k not smaller than zero?

  1. Jul 4, 2016 #1
    Question:

    The equation kx2+kx+3-k = 0, where k is a constant, has no real roots

    a) Show that 5k2-12k < 0
    b) Find the set of possible values of k

    Solution attempted:
    a) For no real roots, b2-4ac < 0

    a = k
    b = k
    c = 3 - k

    k2 - 4k(3 - k) < 0
    k2 -12k + 4k2 < 0
    5k2 - 12k < 0
    Hence shown

    b) 5k2 - 12k < 0
    k(5k-12) < 0
    So k < 0 or (5k-12) < 0, k< 12/5, k<2.4

    So k < 2.4

    But the answer says 0 < k < 2.4

    Why?
     
    Last edited: Jul 4, 2016
  2. jcsd
  3. Jul 4, 2016 #2

    micromass

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    Your ##b## should be squared.
     
  4. Jul 4, 2016 #3
    Done.
    I'm self-studying maths, and it doesn't say why in the book.
     
  5. Jul 4, 2016 #4

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    If k<0, then k(5k-12) < 0 cannot be true because both factors are negative.
     
  6. Jul 4, 2016 #5
    Because if k<0, (5k-12) would be positive :wink:
    got it
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Why is k not smaller than zero?
  1. Why I, J, K ? (Replies: 7)

  2. Why zero? (Replies: 28)

Loading...