2-07.scm

#lang sicp

(define (make-inteval a b) (cons a b))
(define (lower-bound interval) (car interval))
(define (upper-bound interval) (cdr interval))

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