# Lec4 - Solving nonlinear equations Open methods...

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Solving nonlinear equations Open methods

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Lecture 4 2 Newton-Raphson method Fast workhorse for finding roots Step 1: Start with an initial guess x i Step 2: Extrapolate tangent line to x -axis ( y = 0 ) to get next estimate of root. Step 3: Repeat until converges x i x i +1 f ( x i ) x f ( x ) Convergence condition: | x i +1 x i | < ε a tolerance 1 1 ii a i xx x + + < or
Lecture 4 3 Newton-Raphson method x i x i +1 f ( x i ) x f ( x ) x i x i +1 f ( x i ) 1 () i i ii f x fx xx + = 1 i i f x f x + ⇒= Formula for next x i+1 from x i : Slope at x i : Fundamental equation for Newton-Raphson method Note that the method fails if f ’(x i ) ever vanishes!

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Lecture 4 4 Using Newton-Raphson () ( 1 ) x fx e xx =− + 2 1 x f xe x NewtRaphL4('func', 'deriv’, x0, eps, imax) ε a # iterations to converge eps FalsePosL3 NewtRaphL4 x0 [-2. 0.] -2. 10 -3 74 10 -4 94 10 -5 11 5 10 -6 13 5 Newton-Raphson converges at least twice as fast! Example with MatLab …
MatLab work … script1_L4.m Lecture 4 5 z >> x=-2.5:0.2:2.5; z >> y=functL4(x); z >> plot(x,y,[-2.5 2.5],[0 0]) z >> xlabel('x') z >> ylabel('y') z >> grid z >> FalsePosL3('functL4', -2., 0., 1e-3, 20) z >> NewtRaphL4('functL4', 'derivL4', -2., 1e-3, 20) z >> FalsePosL3('functL4', -2., 0., 1e-4, 20) z >> NewtRaphL4('functL4', 'derivL4', -2., 1e-4, 20) z >> FalsePosL3('functL4', -2., 0., 1e-5, 20) z >> NewtRaphL4('functL4', 'derivL4', -2., 1e-5, 20) z >> FalsePosL3('functL4', -2., 0., 1e-6, 20) z >> NewtRaphL4('functL4', 'derivL4', -2., 1e-6, 20)

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Lecture 4 6 Understanding the convergence of Newton-Raphson method 2 11 1 1 () ( ) ( ) ( ) ( ) ( ) [ ,] 2 ii i
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## This note was uploaded on 12/29/2011 for the course CHE 132b taught by Professor Ceweb during the Fall '09 term at UCSB.

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Lec4 - Solving nonlinear equations Open methods...

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