Ex-2.10

(define (div-interval x y)
  (if (or (= (lower-bound y) 0)
          (= (upper-bound y) 0))
      (error ("division by zero"))
      (mul-interval
       x
       (make-interval
        (/ 1.0 (upper-bound y))
        (/ 1.0 (lower-bound y))))))