1/2+1/4-1/5=11/20，-1/3+1/6-1/12=-1/4，11/20-5/20=0.3
1/2-1/3+1/4-1/5+1/6-1/12=6/12-4/12+3/12-1/5+2/12-1/12=(6-4+3+2-1)/12-1/5=5/10-2/10=3/10
