Change of Sign Method.

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Pure Mathematics 2                                                                                        Katie Ruck

Solution of Equations by Numerical Methods

Change of Sign Method

If we let f(x)=3x³-0.5x²-0.5x-1, in order to solve this equation and determine its roots, it is necessary for it to be written in the form 3x³-0.5x²-0.5x-1=0.

The root of the equation f(x)=0 is indicated where y=f(x) crosses the x-axis.

Roots of the equation 3x³-0.5x²-0.5x-1=0, will be found to a three decimal place accuracy.

        Having illustrated the equation graphically using Autograph, it is evident that the equation has only root that lies between the interval [0,1].

        Before proceeding, it is necessary to check that there is a sign change in the above interval:

f(0) = 0-0-0-1= -1

f(1) = 3-0.5-0.5-1 =1

        The method that will be used for the numerical solution of the equation is the decimal search method.

        I will first take the increments in x of size 0.1 within the interval [0,1], calculating the value of the function 3x³-0.5x²-0.5x-1 for each one, until a change of sign is found.


        The above table illustrates that there is a sign change.  It can therefore be understood that the root lies in the interval [0.8,0.9].

        Having narrowed down the interval, I will continue with the decimal search, but now using increments of 0.01 within the interval [0.8,0.9].

        This indicates that the root lies in the new interval [0.83,0.84].


I will continue the process using the increments of 0.001 within the interval [0.83,0.84].


The root therefore lies in the interval [0.838,0.839].  I shall now use increments of 0.0001 in the interval [0.838,0.839].

        Therefore the root lies between 0.8389 and 0.8390.  To three decimal places, the root = 0.839.

Error Bounds

        A change of sign method such as the one used, provides bounds within which a root lies so that the maximum possible error in a result is known.

When x = 0.8385, f(0.8385) = -0.00218772512

When x = 0.8395, f(0.8395) = 0.00280856463

The error bounds of the root 0.839 are 0.839 ± 0.0005.

        However, I am able to say that I have a more accurate solution, as I know that the root lies in the interval [0.8389,0.8390].

Failure of the Change of Sign Method

        There are a number of situations that can cause problems for change of sign methods.  For example, let y=f(x)=x³-2x²-x+2.63.  

        This curve is shown graphically below.

        With this example I shall use the decimal search method to find a change of sign and so investigate the roots of the previous equation.  Integers will be used as the x-values.

        From the graph it is evident that the equation x³-2x²-x+2.63=0 has more than one root, i.e. roots that lie in the intervals [-2,-1] and [1,2].  However, the above table illustrates that the method used only detects one such root that lies in the interval       [-2,-1].

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        In this case, using the method of decimal search has caused an incorrect conclusion to be reached.  This is because the curve touches the x-axis between x=1 and x=2, therefore there is no change of sign and consequently all change of sign methods are doomed to failure.

Fixed Point Iteration Method

        Let y=f(x)=x3+2x2-4x-4.58.  The graph is shown below:

        The roots of the equation can be found where f(x)=0.  From the graph, it is evident that the roots of the equation lie in the intervals [-3,-2],[-1,0] and [1,2].

        Using a sign change search verifies that the intervals within which the roots ...

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