习题2.2

代码如下

(defn make-segment [start-point end-point] (list start-point end-point))
(defn start-segment [segment] (first segment))
(defn end-segment[segment] (last segment))
(defn make-point [x y ](list x y))
(defn x-point [point] (first point))
(defn y-point [point] (last point))
(defn midpoint [segment](
                        make-point (/ (+ (x-point (start-segment segment)) (x-point (end-segment segment)) )  2) (/ (+ (y-point (start-segment segment)) (y-point (end-segment segment)) ) 2)
                         ))
(defn print-point [point] (println) (print "(" (x-point point) "," (y-point point) "