interval arithmetic.ss

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

(define (lower-bound int) (car int))
(define (upper-bound int) (cdr int))

(define (add-interval x y)
  (make-interval (+ (lower-bound x) (lower-bound y))
                 (+ (upper-bound x) (upper-bound y))))

(define (sub-interval x y)
  (make-interval (- (lower-bound x) (upper-bound y))
                 (- (upper-bound x) (lower-bound y))))

(define r1 (make-interval 4.2 4.5))
(define r2 (make-interval 4.2 4.5))

(add-interval (sub-interval r2 r1) r1) ; (3.9000000000000004 . 4.8)