丨1-1/2丨-丨1/2-1/3丨-丨1/3-1/4丨-……-丨1/3-1/4丨
= 丨1-1/2丨-丨1/2-1/3丨-丨1/3-1/4丨
= 1 -1/2 - 1/2 + 1/3 -1/3 + 1/4
= 1 - 1 + 1/4
= 4分之1打错··最后是1/9-1/10