Newton-Raphson Method

Introduction

Consider a graph below of . Let us consider is an approximate root of . Draw a tangent at the curve at as shown in figure aside. The point of intersection of this tangent with the x-axis gives the second approximation to the root. Let the point of intersection be . The slope of the tangent is given by;

   

where is the slope of at . Solving for we obtain

   

This is called the Newton-Raphson formula.
The next approximation would be

   

So, we can generalize the next approximation using

   

And finally the process is repeated until the difference between the consecutive values are within prescribed limit.

Algorithm

Code

C

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#define f(x) ((x * x * x) - 18)
#define fd(x) (3 * x * x)
#define fdd(x) 6 * x
int main()
{
    float x0, x1, error, errorold, converge, order;
    int i = 0;
    printf("Input the approximation : ");
    scanf("%f", &x0);
    converge = (f(x0) * fdd(x0)) / (fd(x0) * fd(x0));
    if (converge > 1)
        exit(1);
    printf("Ite\tX0\t\tX1\t\tError\t\tOrder\n");
    do
    {
        errorold = error;
        x1 = x0 - (f(x0) / fd(x0));
        if (f(x1) == 0)
        {
            break;
        }
        error = fabs(x1 - x0);
        printf("%2d\t%4.6f\t%4.6f\t%4.6f\t", ++i, x0, x1, error);
        if (i == 1 || error == 0 || errorold == 1)
        {
            printf("-----\n");
        }
        else
        {
            order = log(error) / log(errorold);
            printf("%4.6f\n", order);
        }
        x0 = x1;
    } while (error > 0.00005);
    printf("Root is %4.6f", x0);
    return 0;
}

Javascript