丨1/3-1/2丨+丨1/4-1/3丨-丨1/4-1/2丨=丨-1/6丨+丨-1/12丨-丨-1/4丨=1/6+1/12+1/4=2/12+1/12+3/12=（2+1+3）/12=6/12=1/20.5
