练习2.2

(define(make-segment start-point end-point)

(cons start-point end-point))

(define(start-segment seg)

(car seg))

(define(start-segment seg)

     (cdr seg))

(define(make-point x y)

       (cons x y))

(define(x-point p)

(car p))

(define(y-point p)

(cdr p))

(define(average x y)

     (/ (+ x y) 2.0))

(define(mid-segment seg)

       (let ((start (start-segment seg))

         (end (end-segment seg)))

       (make-point (average (x-point start)(x-point end))

                    (average (y-point start)(y-point end)))))

(define(print-point p)

   (newline)

   (display “(“)

   (display (x-point p))

   (display “,”)

   (display (y-point p))

   (display “)”))