1 (require "sicp_2.7-2.11.rkt")
 2 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;2.12
 3 (define (make-center-width c w)
 4   (make-interval (- c w) (+ c w)))
 5 
 6 (define (make-center-percent c p)
 7   (let ((width (* c (/ p 100))))
 8     (make-interval (- c width) (+ c width))))
 9 
10 (define (center i)
11   (/ (+ (lower-bound i) (upper-bound i)) 2))
12 
13 (define (percent i)
14   (* 100 (abs (/ (width-interval i) (center i)))))
15 
16 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;2.14
17 (define (par1 r1 r2)
18   (div-interval (mul-interval r1 r2)
19                 (add-interval r1 r2)))
20 
21 (define (par2 r1 r2)
22   (let ((one (make-interval 1 1)))
23     (div-interval one
24                   (add-interval (div-interval one r1)
25                                 (div-interval one r2)))))
26 
27 (define inter1 (make-center-percent 2 50))
28 (define inter2 (make-center-percent 4 50))
29 (define A (make-center-percent 2 0.0004))
30 (define B (make-center-percent 4 0.0004))
31 
32 (par1 inter1 inter2)
33 (par2 inter1 inter2)
34 
35 (div-interval A B)
36 (div-interval A A)