有一分数序列:2/1,-3/2,5/3,-8/5,13/8,-21/13..求出这个数列的前20项之和。x=1,x=-x
L=[[2,1]] for _ in range(20): L.append([sum(L[-1]),L[-1][0]]) s=0 flag=1 for i in L: s+=flag*i[0]/i[1] flag*=-1 print(s)