function I = rule3_8(f, a, b, N)

if mod(N,3) ~= 0
    fprintf('Error: N has to be divisible by 3\n');
end;

dx = (b - a) / N;
S = f(a) + f(b);

for i = 1 : N-1
    yi = f(a + i * dx);
    if mod(i, 3) ~= 0
        S = S + 3 * yi;
    else
        S = S + 2 * yi;
    end;
end

I = S * 3 / 8 * dx;