2.1(构造序对)

(define (make-rat n d)
  (let ((g (gcd n d)))
    (cons (/ n g) (/ d g))))

(define (numer x) (car x))
(define (denom x) (cdr x))

(define (print-rat x)
  (newline)
  (cond ((= (denom x) 1) (display (numer x)))
      ((display (numer x))
      (display "/")
      (display (denom x)))
      ))

(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? x y)
  (newline)
  (= (* (numer x) (denom y))
     (* (denom x) (numer y))))

(define one-half (make-rat 1 2))
(print-rat one-half)

(define one-third (make-rat 1 3))
(print-rat one-third)

(print-rat (add-rat one-half one-third))
(print-rat (mul-rat one-half one-third))
(print-rat (sub-rat one-half one-third))
(print-rat (div-rat one-half one-third))
(equal-rat? one-half one-third)
(print-rat (add-rat one-third one-third))
(print-rat (make-rat 2 1))
(print-rat (make-rat 4 2))
(print-rat (make-rat 2 2))


;2.1
(print-rat (sub-rat one-third one-half))
(print-rat (sub-rat one-third one-third))