LISP NCNU HW手寫exercise2.1

(define (make-rat n d)
(cond ( (and (< n 0) (< d 0)) (let ((g (gcd n d))) (cons (/ (- n) g) (/ (- d) g))))
     ( (and (> n 0) (< d 0)) (let ((g (gcd n d))) (cons (/ (- n) g) (/ (- d) g))))
     (else (let ((g (gcd n d))) (cons (/ n g) (/ d g))))
)
)
(define (gcd a b)
  (if (= b 0)
      (if (< a 0) (- a)
    a)
      (gcd b (remainder a b))))