SICP 2.1

;;; Assume n and d are never 0, else gcd will return 0, which causes division by zero
(define (make-rat n d)
  (define g (gcd n d))
  (if (or (and (< n 0) (< d 0)) (and (> n 0) (> d 0))) 
      (cons (/ (abs n) g) (/ (abs d) g))
      (cons (/ n g) (/ d g))))
(define (numer x) (car x))
(define (denom x) (cdr x))
(define (print-rat x)
  (display (numer x))
  (display "/")
  (display (denom x)))