Evaluating a double integral using the trapezoidal rule

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Robert
Robert am 22 Feb. 2011
Beantwortet: Mohammed am 12 Dez. 2023
I am trying to take the double integral of the function using the Trapezoidal rule for G=integral (3*x.^2*y+cos(2*x)*sin(y)+2+4*y.^-2*x+5*y)dxdy with x interval 0 to 2pi and y interval 1 to 10. I found a formula for it but don't know the proper syntax to enter it in. We must use the trapezoidal rule because we are comparing different techniques for evaluating integrals. Here is what I have so far:
x1=0;
x2=2*pi;
y1=1;
y2=10;
N=101
dx=(x2-x1)/(N-1);
dy=(y2-y1)/(N-1);
x=x1:dx:x2;
y=y1:dy:y2;
g(x,y)=3*x.^2*y+cos(2*x)*sin(y)+2+4*y.^-2*x+5*y;
N=101;
for i=1:N+1
for j=1:N+1
out(i,j)=((dx*dy)/4)*(g(i,j(1,1))+g(i,j(1,N))+g(i,j(N,1))+g(i,j(N,N))+2*(g(i,j(1,2:1:N-1))+g(i,j(N,2:1:N-1))+g(i,j(2:1:N-1,1))+g(i,j(2:1:N-1,N)))+4*(sum(g(i,j(2:1:N-1,2:1:N-1)))));
end
end
The really long formula is from this source: http://www.math.ohiou.edu/courses/math344/lecture24.pdf on page 2. I feel that I could streamline the code by using more summation but I do not know how to do that.

Akzeptierte Antwort

Andrew Newell
Andrew Newell am 22 Feb. 2011
How about this:
N = 101;
x = linspace(0,2,N)*pi;
y = linspace(1,10,N);
dx = diff(x(1:2));
dy = diff(y(1:2));
[x,y] = meshgrid(x,y);
mat = 3*x.^2.*y+cos(2*x).*sin(y)+2+4*y.^(-2).*x+5.*y;
mat(2:end-1,:) = mat(2:end-1,:)*2;
mat(:,2:end-1) = mat(:,2:end-1)*2;
out = sum(mat(:))*dx*dy/4;

Weitere Antworten (2)

Mohammed
Mohammed am 12 Dez. 2023
∫ 𝑠𝑖𝑛𝑥𝑑𝑥 𝜋 0 = 𝑐𝑜𝑠

Mohammed
Mohammed am 12 Dez. 2023
∫5 1 1/x 𝑑𝑥 = 𝑙𝑛x

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