#lang sicp

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

(define (lower-bound i)
  (car i))

(define (upper-bound i)
  (cdr i))

(define (mul-interval x y)
  (let ((p1 (* (lower-bound x) (lower-bound y)))
	(p2 (* (lower-bound x) (upper-bound y)))
	(p3 (* (upper-bound x) (lower-bound y)))
	(p4 (* (upper-bound x) (upper-bound y))))
    (make-interval (min p1 p2 p3 p4)
		   (max p1 p2 p3 p4))))

(define (div-interval x y)
  (if (<= (* (lower-bound y) (upper-bound y)) 0)
      (error "division by zero")
      (mul-interval
       x
       (make-interval (/ 1.0 (upper-bound y))
                      (/ 1.0 (lower-bound y))))))