# Homework Help: Matlab intergrals

1. May 6, 2010

### fredrick08

1. The problem statement, all variables and given/known data
Let us consider an approximation to an integral. Let f(x) be some continuous function on
[a, b]. We wish to find an approximation for the integral
I = int from a to b of f(x)dx
in the following manner:
Subdivide the interval into N intervals of length h = (b−a)/N. Let xi = ih for i = 0, . . . ,N.
Let
Ij = int from 0 to h of f(xj+t)dt

Find a cubic polynomial Pj (x) that goes through (xj , f(xj)), (xj + h/3,f(xj + h/3)),(xj+2h/3, f(xj+2h/3) and (xj+1,f(xj+1))
We form an approximation for the integral by letting
I=sum(j=0 to N-1) of w0*f(xj)+w1*f(xj+h/3)+w2*f(xj+2h/3)+w3*f(xj+1)
Find these weights, wi.
In 2 peices of code, plot the first three Bessel functions, J0(x), J1(x) and J2(x), on the
interval [0, 20]. The first peice of code should be a MATLAB function BJ(x, n) outputing
the approximation for the integral representation of Jn, given by
Jn(x) =(1/pi)int from 0 to pi of cos(nt − x sin t)dt
using the above method for 100 subdivisions of [0, pi]. The second peice of code should call
the function an produce the required plots with 2000 subdivisions of [0, 20].

Im just gobsmacked with this qn.. as i only started using matlab a couple of days, ago and have no programming experience.

what i have done so far is really no good, but i have no idea.

function [Jn]=BJ(x,n)
N=100;
b=pi;
a=0
h=(b-a)/N;
x=a:h:b;
xj=i*h;
Ij=0;
J0=cos(n*xj-x*sin(xj));
J1=cos(n*(xj+h/3)-x*sin(xj+h/3));
J2=cos(n*(xj+(2*h/3))-x*sin(xj+(2*h/3)));
J3=cos(n*(xj+1)-x*sin(xj+1));
for i=0:N;
Ij=Ij+w0*J0+w1*J1+w2*J2+w3*J3;
J=(1/pi)*Ij;
end