丨1/3-1/2丨+丨1/4-1/3丨+……+丨1/10-1/9丨=(1/2-1/3)+(1/3-1/4)+……+(1/9-1/10)=1/2-1/3+1/3-1/4+……+1/9-1/10=1/2-1/10=2/5
