compute a scalar r that is the sum of the first 3 elements of matrix x in matlab

To compute a scalar r that is the sum of the first 3 elements of a matrix x in MATLAB, you can use the sum function along with indexing. Here's an example:

main.m
x = [1, 2, 3; 4, 5, 6; 7, 8, 9]; % example matrix
r = sum(x(1:3)); % compute sum of first 3 elements
disp(r); % display the result
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In this example, x is a 3x3 matrix, and we're using the sum function to compute the sum of the first 3 elements of x. We do this by indexing the first three elements using x(1:3), and then passing that to the sum function. Finally, we store the result in the scalar variable r and display it using the disp function.

Hope this helps!