I think I know why but I can't represent this accurately with a graph.
I am supposed to show why Simpson's Rule is so far off for the integral $\int_0^{20} \cos \pi x$
I know that the answer should be zero because it repeats on that interval, 10 up and 10 down evenly.
I know that the antiderivative $\sin \pi x$ will be zero for any 0 or pi value so that evaluates to zero.
When I use Simpson's Rule I get
$\frac{2}{2} (1 + 2 + 2 + 2 + 2 + 2 + 2 + 2 + 2 +1) = 9$
I am pretty certain that the reason the numbers are so far off is because the interval is 2 and that will cover an up and down which Simpson's Rule will overestimate but I can not show this on a graph to equal 9.