令
m = 1/2+1/4+1/8+1/16+1/32+1/64+1/128
两边同时乘2,得：
2m = 1+1/2+1/4+1/8+...+1/64
两式相减,
得：
m = 1 - 1/128 = 127 / 128
或者
1/2+1/4+1/8+1/16+1/32+1/64+1/128
=（1-1/2)+(1/2-1/4)+(1/4-1/8)+...+(1/64-1/128)
= 1 -1/2+1/2 - 1/4 + 1/4 - 1/8 +...+ 1/64 - 1/128
= 1 - 1 / 128
=127 / 128