Results 1 to 6 of 6

Math Help - Polynomial and Rational functions - Descartes Rule Of Signs

  1. #1
    Newbie
    Joined
    Jun 2011
    Posts
    8

    Polynomial and Rational functions - Descartes Rule Of Signs

    Sometimes finding the factors of the polynomial function can be very time consuming. The french mathematician rene descartes discovered that if a polynomial function f(x) has real coefficients and is written in decreasing degree, then the number of positive real zeros that the function has is the same as the number of sign changes in f(x) as you follow it from term to term or is less than that by multiple of two. The number of negative real seros that the function has is the same as the number of sign changes in the polynomial functon f(-x) or is less that that by a multiple of two. This is known as Descartes Rule of Signs.

    Consider the polynomial function: f(x) = x^5 - 5x^4 + 3x^3 + 17x^2 - 28x + 12

    a) how many sign changes are there in f(x)?
    B)According to Descartes rule of signs how many positive real zeroes might f(x) have?
    c)determine f(-x)
    d)how many sign changes are there in f(-x)
    e)According to descartes rule of signs how many negative real zeroes might f(x) have?
    f) based on your conclusions in (b) and (e) use synthetic division to factor f(x).
    G) draw a sketch of f(x)
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Super Member Quacky's Avatar
    Joined
    Nov 2009
    From
    Windsor, South-East England
    Posts
    901
    There are plenty of sites available to help you understand the rule of signs. How much are you able to do? Where are you struggling? I think the questions are quite straightforward.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Newbie
    Joined
    Jun 2011
    Posts
    8
    im struggling at understanding it i have no clue how to get the zeroes, because arent they in the denominator there is no denomintor??
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Super Member Quacky's Avatar
    Joined
    Nov 2009
    From
    Windsor, South-East England
    Posts
    901
    f(x) = x^5 - 5x^4 + 3x^3 + 17x^2 - 28x + 12

    a) How many sign changes?
    I'll colour them:

    f(x) = + x^5 - 5x^4 + 3x^3 + 17x^2 - 28x + 12

    That is four sign changes.

    b) How many positive real roots?
    There could be up to four. That is what is significant about the rule of signs. There could be two, or there could be none, but the maximum is four.

    Can you try the other parts?
    Follow Math Help Forum on Facebook and Google+

  5. #5
    Newbie
    Joined
    Jun 2011
    Posts
    8
    why did u colour the + signs i dont understand why?
    Follow Math Help Forum on Facebook and Google+

  6. #6
    MHF Contributor

    Joined
    Apr 2005
    Posts
    16,214
    Thanks
    1789
    Because the problem asked for the number of sign changes. By coloring the "+" signs differently from the "-" signs, Quacky was trying to help you count the number of changes. Can you not at least count them?
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Using Descartes' Rule of Signs
    Posted in the Pre-Calculus Forum
    Replies: 5
    Last Post: May 21st 2010, 01:47 PM
  2. Descartes rule of signs
    Posted in the Pre-Calculus Forum
    Replies: 4
    Last Post: March 19th 2009, 06:50 AM
  3. [SOLVED] Descartes's Rule Of Signs
    Posted in the Pre-Calculus Forum
    Replies: 5
    Last Post: October 4th 2008, 03:56 PM
  4. Descartes Rule of Signs
    Posted in the Algebra Forum
    Replies: 1
    Last Post: May 6th 2008, 07:36 PM
  5. Descartes' Rule of Signs Help
    Posted in the Algebra Forum
    Replies: 1
    Last Post: January 17th 2008, 10:45 AM

Search Tags


/mathhelpforum @mathhelpforum