SICP Exercise 1.12

gistfile1.txt

#lang scheme

(define (fact n)
 (fact-tail n 1))

(define (fact-tail n acc)
  (if (= 0 n)
      acc
      (fact-tail (- n 1) (* acc n))))

(define (pascal k n)
  (cond
    ((= k 0) 1)
    ((= k n) 1)
    ((> k n) 0)
    (else
     (/ (fact n) (* (fact k) (fact (- n k)))))))