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www.le.ac.uk
Numerical Methods: Finding Roots
Department of MathematicsUniversity of Leicester
Content
Motivation
Change of sign method
Iterative method
Newton-Raphson method
Reasons for Finding Roots by Numerical Methods• If the data is obtained from observations,
it often won’t have an equation which accurately models
• Some equations are not easy to solve
• Can program a computer to solve equations for us
Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
Solving equations by change of sign
• This is also known as ‘Iteration by Bisection’
• It is done by bisecting an interval we know the solution lies in repeatedly
Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
METHOD
• Find an interval in which the solution lies
• Split the interval into 2 equal parts
• Find the change of sign
• Repeat
Solving equations by change of sign
Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
Solving equations by change of sign
Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
Example: Find a root of the equation
given there is a solution close to x=-2
Step 1: Find the interval
So we know the solution lies between -2 and -1
Solving equations by change of sign
Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
Step 2: We now half the interval and find the
value of f at the half way point
Now we know the solution lies between and
Solving equations by change of sign
Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
𝑓 (−1.5 )=2 (−1.5 )3−2 (−1.5 )+7=6.625
Step 3: Now we just keep repeating the process
Solving equations by change of sign
Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
Solving equations by change of sign
Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
So to 3 s.f. the solution is
Solving equations by change of sign
Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
𝑥=−1.74
Solving equations by change of sign
Number of dp:
Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
Find a solution
Clear information box
Solving using iterative method
• ‘Iteration’ is the process of repeatedly using a previous result to obtain a new result
Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
METHOD
• Rearrange the equation to make the highest power the subject
• Use the power root to leave on its own on the LHS
• Make on the LHS
• Make on the RHS
Solving using iterative method
Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
• Now that the function is in the form
we can use the value for to calculate , then we can use the value , and so on...
• When we eventually get a value repeating we have reached the solution
Solving using iterative method
Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
𝑥𝑛+1= 𝑓 (𝑥𝑛)
Solving using iterative method
Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
Solving using iterative method
Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
Solving using iterative method
Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
Click on a seed value to see the cobweb:
start
here
start
here
start
here
start
here
start
here
start
here
𝑦=𝑥
𝑦= 𝑓 (𝑥 )
Clear Cobwebs
Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
Example: Find a root of the equation
given that there is a solution close to
STEP 1: Rearrange the equation
Solving using iterative method
Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
Step 2: We can now input (taken from the
question)
Solving using iterative method
Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
This gives us the solution
to 3 d.p.
Solving using iterative method
Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
𝑥=−1.893
Solving using iterative method
Starting value:
Number of d.p.:
Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
Solve
Clear
Newton-Raphson Method
• Sometimes known as the Newton Method
• Named after Issac Newton and Joseph Raphson
• Iteratively finds successively better approximations to the roots
Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
Newton-Raphson Method
The formula is
We start with an arbitrary and wait for the
iteration to converge
Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
𝑥𝑛+1=𝑥𝑛−𝑓 (𝑥𝑛)𝑓 ′ (𝑥𝑛)
Newton-Raphson Method
𝑥0𝑥1𝑥3
Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
Newton-Raphson Method
Example: Use the Newton-Raphson Method to
approximate the cube root of 37
The equation we use is
Now we need to evaluate
Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
Newton-Raphson Method
We then obtain the formula
Choose
Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
𝑓 (𝑥𝑛+1 )=𝑥𝑛−(𝑥𝑛)3−373 (𝑥𝑛 )2
Newton-Raphson Method
So this means that the cube root of 37 is approximately 3.3322 Next
Iterative method
Newton-Raphson
Change of sign methodMotivation
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