2_2.scm

#lang sicp

(define (average x y)
  (/ (+ x y) 2))

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

(define (start-segment seg)
  (car seg))

(define (end-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 (midpoint-segment seg)
  (make-point (average (x-point (start-segment seg))
                       (x-point (end-segment seg)))
              (average (y-point (start-segment seg))
                       (y-point (end-segment seg)))))

(define (print-point p)
  (newline)
  (display "(")
  (display (x-point p))
  (display ",")
  (display (y-point p))
  (display ")"))

(define seg (make-segment (make-point 5 6) 
                           (make-point 7 12))) 
  
(print-point (midpoint-segment seg))