原式可写为
4[(1-1/5)+(1/5-1/9)+(1/9-1/13)+……+(1/97-1/101)]
=4(1-1/5+1/5-1/9+1/9-1/13+……+1/97-1/101)
=4(1-1/101)
=4*100/101
=400/101