1/42=1/(6*7)=1/6-1/7
1/56=1/(7*8)=1/7-1/8
1/72=1/8-1/9
.
1/132=1/11-1/12
1/42+1/56+1/72+1/90+1/110+1/132=1/6-1/12=1/12