1.31 SICP

iterative-product.scm

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

(define (inc x) (+ x 1))
(define (identity x) x)

(define (factorial n)
  (product identity 2 inc n))

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

(define (pi-approx n)
  (exact->inexact
   (* 4
     (/ (* 2 (product
	      (lambda (x)
		(if (= x n) (* 2 x) (square (* 2 x)))) 2 inc n))
	(product (lambda (x) (square (+ (* x 2) 1))) 1 inc (- n 1))))))