Ex-2.2

(define (make-point x y)
  (cons x y))
(define (x-point p)
  (car p))
(define (y-point p)
  (cdr p))

(define (make-segment p1 p2)
  (cons p1 p2))
(define (start-segment s)
  (car s))
(define (end-segment s)
  (cdr s))

(define (midpoint-segment s)
  (make-point
   (/ (+ (x-point (start-segment s))
         (x-point (end-segment s))) 2)
   (/ (+ (y-point (start-segment s))
         (y-point (end-segment s))) 2)))