sicp 1.29

(define s-rule
  (lambda (f a b n)
    (let [(h (/ (- b a) (* n 1.0)))
          (k 0)]
      (* (/ h 3)
        (sum (lambda (x)
               (let ([k (cond
                          [(= x 0) 1]
                          [(= x n) 1]
                          [(odd? x) 4]
                          [else
                            2])])
                 (* k (f (+ a (* x h))))))
          0 (lambda (x)
              (+ x 1))
          n)))))