연습문제 2.10

(define (div-interval x y)
  (cond ((= (upper-bound y) 0) (error "Divided by Zero!"))
        ((= (lower-bound y) 0) (error "Divided by Zero!"))
        (else
         (mul-interval x
                (make-interval (/ 1.0 (upper-bound y))
                               (/ 1.0 (lower-bound y)))))))