Dan's Thoughts Thinking somewhat carefully

(define (make-center-percent value p)
  (make-center-width value (* value (/ p 100.0))))
(define (percent int)
  (* 100 (/ (width int) (center int))))
 
(define (make-interval a b) 
  (cons a b))
(define (lower-bound int) (car int))
(define (upper-bound int) (cdr int))
(define (make-center-width c w)
  (make-interval (- c w) (+ c w)))
(define (center i)
  (/ (+ (lower-bound i) (upper-bound i)) 2))
(define (width i)
  (/ (- (upper-bound i) (lower-bound i)) 2))
 
(define (add-interval x y)
  (make-interval (+ (lower-bound x) (lower-bound y))
                 (+ (upper-bound x) (upper-bound y))))
 
(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 (div-interval x y)
  (define (spans-zero? i)
    (and 
      (not (> (lower-bound i) 0)) 
      (not (< (upper-bound i) 0)))) 
  (if (spans-zero? y)
    (error "The dividing interval cannot span 0.")
    (mul-interval x 
                  (make-interval (/ 1.0 (upper-bound y))
                                 (/ 1.0 (lower-bound y))))))
 
(define (sub-interval2 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 (sub-interval x y)
  (make-interval (- (lower-bound x) (upper-bound y))
                 (- (upper-bound x) (lower-bound y))))
 
(define (print int)
  (newline)
  (display "[")
  (display (lower-bound int))
  (display ",")
  (display (upper-bound int))
  (display "]")
  (newline))