# The width of an interval

The width of an interval is half of the difference between its upper and lower bounds. The width is a measure of the uncertainty of the number specified by the interval. For some arithmetic operations the width of the result of combining two intervals is a function only of the widths of the argument intervals, whereas for others the width of the combination is not a function of the widths of the argument intervals. Show that the width of the sum (or difference) of two intervals is a function only of the widths of the intervals being added (or subtracted). Give examples to show that this is not true for multiplication or division.

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``````(define (make-interval a b)
(cons a b))

(define (lower-bound interval)
(car interval))

(define (upper-bound interval)
(cdr interval))

(make-interval (+ (lower-bound a) (lower-bound b))
(+ (upper-bound a) (upper-bound b))))

(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)
(mul-interval x
(make-interval (/ 1.0 (upper-bound y))
(/ 1.0 (lower-bound y)))))

(define u 15)
(define l -5)

(define u2 10)
(define l2 5)

(define interval (make-interval l u))
(define interval2 (make-interval l2 u2))

(define interval-mul (mul-interval interval interval2))

(define interval-div (div-interval interval interval2))

(check-equal? (width interval) (/ (abs(- l u)) 2))

(check-equal? (width interval-add) (+ (width interval)
(width interval2)))

(check-not-equal? (width interval-mul) (* (width interval)
(width interval2)))

(check-not-equal? (width interval-div) (/ (width interval)
(width interval2)))``````