Change of Sign Method - Mathematical Essay

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Change of Sign Method - Mathematical Essay

In order to find the roots of an equation that cannot be solved

algebraically, I can use numerical methods to do this instead. One of

these methods is the change of sign method. From looking at a graph of

my equation I can find two integers that my root lies between, then

from there, using spreadsheets, I can use the change of sign method to

discover where the root lies to five decimal places.

I have chosen to try to solve the equation: 5x3-7x+1=0

First, I drew the graph of y=5x3-7x+1 in autograph to find where the

roots roughly lie by looking where the graph cuts the x-axis.

Y=5x3-7x+1

[IMAGE]

From this graph I can see that there are three roots, in the intervals

[-2, -1], [0, 1] and [1, 2]. Looking at the root in the interval [1,

2], we bisect this interval and find the midpoint, 1.5.

f(1.5) = 7.375, so f(1.5)> 0. Since f(1) <0, the root is in [1,

1.5].

Now I am going to take the midpoint of this second interval, 1.25.

f(1.25) = 2.016, so f(1.25)> 0. Since f(1) <0, the root is in [1,

1.25].

The midpoint of this reduced interval is 1.125.

f(1.125) = 0.244, so f(1.125)> 0. Since f(1) <0, the root is in [1,

1.125].

The method then continues in this manner until the required degree of

accuracy is achieved. However, this takes a long time to converge the

root, and can be solved more rapidly using a spreadsheet.

To put my data into a spreadsheet, I first needed to design one that

achieved the desired purpose: finding the roots of the equation using

the change of sign method. Below, the formula which I typed into my

spreadsheet are shown:

Change of sign method

y=5x^3-7x+1

a

b

(a+b)/2

f(a)

f(b)

f((a+b)/2)

1

2

=(A5+B5)/2

=5*A5^3-7*A5+1

=5*B5^3-7*B5+1

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