1/12+9/20-1/30-13/42+1/56
=1/3-1/4+1/4+1/5-1/5+1/6-1/6-1/7+1/7+1/8
=1/3+1/8
=11/24
1/3+3/4+2/5+5/7+7/8+9/20+10/21+11/24+19/35
=1/3+3/4+2/5+5/7+7/8+1/4+1/5+1/3+1/7+1/3+1/8+1/5+2/7
=(1/3+1/3+1/3)+(3/4+1/4)+(2/5+1/5+1/5)+(5/7+1/7+2/7)+(7/8+1/8)
=1+1+4/5+8/7+1
=4又33/35