1 (define (add-interval x y)
2   (make-interval (+ (lower-bound x) (lower-bound y))
3 		 (+ (upper-bound x) (upper-bound y))))
4 (define (mul-interval x y)
5   (let ((p1 (* (lower-bound x) (lower-bound y)))
6 	(p2 (* (lower-bound x) (upper-bound y)))
7 	(p3 (* (upper-bound x) (lower-bound y)))
8 	(p4 (* (upper-bound x) (upper-bound y))))
9     (make-interval (min p1 p2 p3 p4)
10 		   (max p1 p2 p3 p4))))
11 
12 (define (div-interval x y)
13   (mul-interval x
14 		(make-interval (/ 1.0 (upper-bound y))
15 			       (/ 1.0 (lower-bound y)))))
16 
17 (define (make-interval a b)
18   (cons a b))
19 (define (upper-bound x)
20   (cdr x))
21 (define (lower-bound x)
22   (car x))
23 
24