急 ! F5 sequence 1題唔明 #35b

請詳細解釋下題原因 :


圖片參考：https://s.yimg.com/rk/HA05788109/o/1517071518.jpg


(b)答案 :The x coordinate of the final position ofthe ant= 100 / [1 - (-1/4)]= 80The y coordinate of the final position ofthe antThe first term = 50 [為甚麼這個不是200而是50 ? 而上部份也是用100]The common ratio = -1/4 The y coordinate of the final position ofthe ant= 200 - {50 / [1 - (-1/4)]} [為甚麼要用200 - ??]= 160The coordinates of the final position ofthe ant= (80, 160)