1/2,1/3,1/4,1/5,…,1/90,1/91,你能从中挑出十个,加上正负号,使它们的和等于-1
问题描述：
1/2,1/3,1/4,1/5,…,1/90,1/91,你能从中挑出十个,加上正负号,使它们的和等于-1
答
-1=-1/2-1/2=-(1/3+1/6)-(1/4+1/4)
=-(1/2-1/6+1/12+1/12)-(1/5+1/20+1/8+1/8)
=1/6-1/2-1/12-1/36-1/18-1/5-1/20-1/8-1/40-1/10
答
解这种题,一般首先是找出满足结果的和,不管有多少组,然后剩下的可以通过对消得0.-1/2-1/3-1/6=-1还差7个数1/4-(1/8+1/12+1/24)=01/10-(1/15+1/30)=0=>-1/2-1/3-1/6-1/4+1/8+1/12+1/24-1/10+1/15+1/30=-11/8-(1/12+1/...