Quadratic equation

Discussion in 'Math' started by jagjit Sehra, Mar 14, 2008.

  1. jagjit Sehra

    Thread Starter Member

    Feb 29, 2008
    25
    0
    Hi Folks,
    I am having some difficulties in understanding what my graph is showing me in MATLAB.
    I have chosen the value for a,b and c for the quadratic so the result will be complex, however the graph does not point to the values when the function cross the 0 axis. the first one does but according to the results the second should be in the third quadrant.

    a = 1;
    b = 0.009;
    c = 5;
    x =linspace(-16,16);
    y = a*x .* x + b*x + c
    plot(x,y)
    x1=(-b+sqrt((b^2)-(4*a*c)))/(2*a)
    x2=(-b-sqrt((b^2-4)*(a*c)))/(2*a)

    The two roots are as follows:

    x1 = -0.0045 + 2.2361i

    x2 = -0.0045 - 2.2360i



    x1= seems to be correct, however x2 is somewhere in the third quadrant this is not reflected on the graph generated by MATLAB.
    I know this is something I am doing wrong I would appreciate it, if someone will show me the error of my ways.
     
  2. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    144
    x2 should be:

    Code ( (Unknown Language)):
    1. x2=(-b-sqrt((b^2)-(4*a*c)))/(2*a)
    Dave
     
  3. jagjit Sehra

    Thread Starter Member

    Feb 29, 2008
    25
    0
    Thanks Dave,
    I didnt spot that Shame on me!
    Jag.
     
  4. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    144
    Did you consider the Matlab roots function?

    For:

    a = 1;
    b = 0.009;
    c = 5;

    Code ( (Unknown Language)):
    1. p = [1 0.009 5];
    2.  
    3. q = roots(p)
    4.  
    5. q =
    6.  
    7.   -0.0045 + 2.2361i
    8.   -0.0045 - 2.2361i
    Dave
     
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