1/1*5+1/5*10+1/10*15+...+1/95*100
=1/5+(1/5-1/10)/5+(1/10-1/15)/5+.+(1/95-1/100)/5
=1/5+(1/5-1/10+1/10-1/15+.+1/95-1/100)/5
=1/5+(1/5-1/100)/5、
=1/5+19/500
=119/500
1/1*5+1/5*10+1/10*15+...+1/95*100
=1/5+(1/5-1/10)/5+(1/10-1/15)/5+.+(1/95-1/100)/5
=1/5+(1/5-1/10+1/10-1/15+.+1/95-1/100)/5
=1/5+(1/5-1/100)/5、
=1/5+19/500
=119/500