Nima - perhaps try
for n = [20,50,100,150,400]
f = @(x)6*(cos(x/2)) .* (sin(x/2)).^2; % Defining given function
a = 0; % lower limit
b = pi; % upper limit
h = (b-a)/n; % Width of each interval
s = 1/2 * (f(a) + f(b)); % Summation value
% make for loops to calculate values between the intervals
for i = 1 : n-1
s = s + f(a + i*h);
end
A = h * s; % The calcuated sum of all intervals
format long % Make the outcome to be in long format
fprintf('n = %d value=%f\n', n, A); % Display the outcome
end
I've replaced the call to display with fprintf to write out the value of n and A.
