11/20-19/30+29/42-41/56+55/72-71/90+89/110-109/132,要有过程

=(1-9/20)+(-1+11/30)+(1-13/42)+(-1+15/56)+(1-17/72)+(-1+19/90)+(1-21/110)+(-1+23/132)
=-9/20+11/30-13/42+15/56-17/72+19/90-21/110+23/132 正负1抵消
=-(1/4+1/5)+(1/5+1/6)-(1/6+1/7)+(1/7-1/9)-(1/9-1/11)+(1/11+1/12)
=-1/4+1/12
=-1/6
如19/90=(10+9)/(10×9)=1/9+1/10 其它类似
特点(x+y)/xy=1/x+1/y