1 (define (par1 r1 r2)
2   (div-interval (mul-interval r1 r2)
3 		(add-interval r1 r2)))
4 (define (par2 r1 r2)
5   (let ((one (make-interval 1 1)))
6     (div-interval one
7 		  (add-interval (div-interval one r1)
8 				(div-interval r2)))))
9 
10 (define a (make-center-percent 100 5))
11 (define b (make-center-percent 200 2))
12 (define aa (div-interval a a))
13 (define ab (div-interval a b))
14 (center aa)
15 (center ab)
16 (percent aa)
17 (percent ab)
18 (define apb1 (par1 a b))
19 (define apb2 (par2 a b))
20 apb1
21 apb2
22 (define apa1 (par1 a a))
23 (define apa2 (par2 a a))