1. Not finding help here? Sign up for a free 30min 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!

Quadratic functions

  1. Feb 12, 2007 #1
    1. A quadratic fraction is of the form f(x)= 2x^2+bx+5
    Find the values for b where the graph of f(x):
    1.just touches the x-axis
    2. has two x-intercepts
    3.does not cut or touch the x-axis


    I've tried using the quadratic formula but i have no idea how to do it when there are two unknowns. For number one i thought it must be when y=0 and i've tried substituting other numbers in but other than that i can't start. Can you please start me off?
     
  2. jcsd
  3. Feb 12, 2007 #2
    When you apply the quadatic formula what are you getting? If [tex] x_{12} = \frac{-b \pm \sqrt{b^2-4ac}}{2a} [/tex] then what does it mean if [tex] x_{12} [/tex] is complex? What about if [tex] x_{12} [/tex] is real? What condition yields a [tex] x_{12} [/tex] as complex? and which condition yields [tex] x_{12} [/tex] as real?
     
  4. Feb 12, 2007 #3
    b has to be a real number but how can you work it out when there are two unknowns .. x is unknown and so is b
     
  5. Feb 12, 2007 #4

    danago

    User Avatar
    Gold Member

    Using the quadratic formula FrogPad posted, pay special attention to the [tex]b^2-4ac[/tex] part.
     
  6. Feb 12, 2007 #5

    cristo

    User Avatar
    Staff Emeritus
    Science Advisor

    You do not need to look at the complete quadratic formula, just the discriminant b2-4ac. Do you know the condition on the discriminant for the equation to have (i)two real roots; (ii) one real repeated root; (iii) no real roots (equivalent to saying the equation has complex roots)?
     
    Last edited: Feb 12, 2007
  7. Feb 12, 2007 #6
    Yes, but that is before you take into account the 3 subtasks. If a graph touches the x-axis once, how does that affect the number of real solutions to the equation?
     
  8. Feb 12, 2007 #7
    ooooooook so...
    1. b=square root of 40
    2 b is greater than square root of 40
    3. b is less than square root of 40

    is that right?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Quadratic functions
  1. Quadratic function (Replies: 4)

Loading...