compute a scalar r that is the sum of the first 20 elements plus the last 5 elements of x in matlab

Assuming that x is a row vector, we can use MATLAB's indexing and sum functions to compute the scalar r as follows:

main.m
x = <your_vector>;  % replace <your_vector> with the name of your vector
r = sum(x(1:20)) + sum(x(end-4:end));
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Here, we are using x(1:20) to extract the first 20 elements of x, x(end-4:end) to extract the last 5 elements of x, and sum() to compute the sum of each sub-vector. Finally, we add the two sums to obtain the scalar r.