sicp 2.13

sicp_2_13

(define (make-interval a b) (cons a b))
 
(define (upper-bound x) (car x)) 
(define (lower-bound x) (cdr x)) 
(define (width x) (- (cdr x) (car x)))
(define (center i)
   (/ (+ (lower-bound i) (upper-bound i)) 2))

(define (make-center-percent c p)
  (let ((w (* c (/ p 100))))
    (make-interval (- c w) (+ c w))))

(define (percent x)
   (abs (* 100 (/ (- (upper-bound x) (lower-bound x)) (+ (upper-bound x) (lower-bound x)))))
  )

(define (mul-percent x y)
   (abs (* 100 (+ (/ (width x) (center x)) (/ (width x) (center y)))))
  )

(mul-percent (make-interval 95 105) (make-interval 95 105))