Ex-2.1

(define (make-rat n d)
  (let ((n1 (if
             (or (and (< n 0) (< d 0))
                 (and (> n 0) (> d 0)))
             (abs n) (* -1 (abs n))))
        (d1 (abs d))
        (g (gcd n d)))
    (cons (/ n1 g) (/ d1 g))))