1/2+1/3-1/5-1/6
=1/2+(1/3-1/6)-1/5
=1/2+1/6-1/5
=15/30+5/30-6/30
=14/30
=7/15
假如是：1/2-1/3+1/5-1/6
=1/2-(1/3+1/6)+1/5
=1/2-(3/6)+1/5
=1/2-1/2+1/5
=1/5
17/72-19/90+21/110-23/132
=（1/8+1/9）-（?）+（?)-(1/11+1/12)
=（1/8+1/9）-（1/9+1/10）+（1/10+1/11)-(1/11+1/12)
=1/8+(1/9-1/9)-(1/10-1/10)+(1/11-1/11)-1/12
=1/8-1/12
=3/24-2/24
=1/24