answer for sicp question 2.11

sicp-2.11.scm

(define (make-interval a b) (cons a b))

(define (upper-bound r) (max (car r) (cdr r)))

(define (lower-bound r) (min (car r) (cdr r)))

(define (mul-interval x y)
  (define (cross-zero? a b)
    (and (<= a 0) (>= b 0)))
  (let ((a (lower-bound x))
	(b (upper-bound x))
	(c (lower-bound y))
	(d (upper-bound y)))
    (cond ((cross-zero? a b)
	   (cond ((cross-zero? c d)
		  (make-interval (min (* b c) (* a d))
				 (max (* a c) (* b d))))
		 (else
		  (let ((factor (if (< c 0) c d)))
		    (make-interval (* a factor) (* b factor))))))
	  ((cross-zero? c d)
	   (let ((factor (if (< a 0) a b)))
	     (make-interval (* factor c) (* factor d))))
	  ((< (* a c) 0)
	   (make-interval (* a d) (* b c)))
	  (else
	   (make-interval (* a c) (* b d))))))