(define (square x) (* x x))

(define (product-rec term a next b)
  (if (> a b)
      1
      (* (term a)
	 (product term (next a) next b))))


(define (product-iter term a next b)
  (define (iter a result)
    (if (> a b)
	result
	(iter (next a) (* result (term a)))))
  (iter a 1))

(define product product-iter)
;(define product product-rec)

(define (fact n)
  (product
    (lambda (x) x)
    1
    (lambda (x) (+ x 1))
    n))

; Approximation to pi/4
(define (pi-prod n)
  (product
    (lambda (a) (/ (* a (+ a 2)) (square (+ a 1))))
    2
    (lambda (a) (+ a 2))
    n))