(define (gcd a b) (if (= b 0) a (gcd b (remainder a b))))
(define (make-rat n d)
(let ((g (gcd n d)))
(let ((ng (/ n g)) (dg (/ d g)))
(if (< ng 0)
(cons (- ng) (- dg))
(cons ng dg)))))
(define (numer x) (car x))
(define (denom x) (cdr x))
(define (print-rat x)
(display (numer x))
(display "/")
(display (denom x))
(newline))
(define (add-rat x y)
(make-rat (+ (* (numer x) (denom y))
(* (numer y) (denom x)))
(* (denom x) (denom y))))
(define (sub-rat x y)
(make-rat (- (* (numer x) (denom y))
(* (numer y) (denom x)))
(* (denom x) (denom y))))
(define (mul-rat x y)
(make-rat (* (numer x) (numer y))
(* (denom x) (denom y))))
(define (div-rat x y)
(make-rat (* (numer x) (denom y))
(* (denom x) (numer y))))
(define (equal-rat?)
(= (* (numer x) (denom y))
(* (numer y) (denom x))))
(define one-half (make-rat 1 2))
(define one-third (make-rat 1 3))
(define one-mthird (make-rat 1 -3))
(define two-third (make-rat 2 3))