Integral using Simpson's Rule
When you call sum, make sure you start with 1 (and not 0).  Otherwise, you get an error of + 2 y_0 h / 3.  In case of (simpsons-rule cube 1 2 1000), this error is 0.000666....

Another way to rewrite the series is to group even and odd terms together, excluding the first and last terms.

(h / 3) * (y_0 + y_n + 4 * (y_1 + y_3 + ... + y_n-1) + 2 * (y_2 + y_4 + ... + y_n-2))

This gives us another possible implementation:

(define (simpsons-rule f a b n)
  (define h (/ (- b a) n))
  (define (y-k k) (f (+ a (* k h))))
  (define (next n) (+ n 2))
  (* (/ h 3.0) (+ (y-k 0) (y-k n) (* 4 (sum y-k 1 next (- n 1))) (* 2 (sum y-k 2 next (- n 2))))))