中间两项和为10，开头和结尾的为-10，结果为0
答案是 -7-5/6=47/6
全部抵消，0
(6)= -2/3+31/6+29/6-28/3 =-4/6+31/6+29/6-56/6 =0
(6)= -2/3+31/6+29/6-28/3 =-4/6+31/6+29/6-55/6 =27/6+29/6-55/6 =1/6
