sicp 2.12

sicp_2_12

(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 (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 (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)))))
  )

(make-center-percent 100 5)

(percent (make-interval 95 105))