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MATLAB Help

  1. Sep 17, 2004 #1
    Approximate the integral on f(x)=exp(-x) on the interval x=[1,5]. Choose N=10, 100, 1000.

    Okay here is what I did:
    % a,b limits of integration

    % x variable of integration

    % f integrand

    % N is the number of sub-intervals

    % h width of each sub-interval

    % TL, TR contribution from left and right endpoints

    %TI contribution from the intermediate points

    a= 1;

    b= 5;

    N = 10;

    h=(b-a)/N;

    x=[a:h:b];

    TL=exp(-a);

    TR=exp(-b);

    TI=0;

    for n=2:N

    f = exp(-n) * x;

    TI=TI+f;

    end
    I=(TL+2*TI+TR)*h*.5

    I =

    Columns 1 through 5

    0.1606 0.1948 0.2291 0.2633 0.2976

    Columns 6 through 10

    0.3318 0.3661 0.4003 0.4346 0.4688

    Column 11

    0.5031

    Aren't I suppose to get only one answer? Why am I getting all this?
     
  2. jcsd
  3. Sep 21, 2004 #2
    My experience with MATLAB is very limited but by the looks of it you have defined x as a row matrix, meaning that f will also be a row matrix and so will TI since your answer is a function of TI yo uare bound to end up with a row matrix. I haven't really looked at the maths involved, what method for integrating did you use ?
     
  4. Sep 21, 2004 #3

    enigma

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The problem arises here:
    f = exp(-n) * x;

    You're multiplying exp(-n) which is a scalar times x which is a vector.

    To fix this you want to access the individual cell of the x vector.

    Try replacing f = exp(-n) * x; with f = exp(-n) * x(n);
     
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