1×5分之1+5×9分之1+9×13分之1+……+97*101分之1
=1/4*[1×5分之4+5×9分之4+9×13分之4+……+97*101分之4]
=1/4[(1/1-1/5)+(1/5-1/9)+(1/9-1/13)+……+1/97-1/101]
=1/4[1/1-1/5+1/5-1/9+1/9-1/13+……+1/97-1/101]
=1/4[1/1-1/(101)]
=1/4*100/101
=25/101