20=4*5
30=5*6
...
132=11*12
1/20 = 1/(4*5)=1/4 - 1/5
1/30 =1/5 - 1/6
1/132 =1/11 - 1/12
左边相加是原式= 右边相加=1/4 - 1/12 =1/6