请解:1/2+1/3+3/4+2/5+5/7+7/8+9/20+10/21+11/24+19/35

因为9/20＝1/4＋1/5,10/21＝1/3＋1/7,
11/24＝1/3＋1/8,19/35＝2/5＋1/7.
所以
原式
= 1/2 +1/3＋3/4＋2/5＋5/7＋7/8＋（1/4＋1/5）＋（1/3＋1/7）＋（1/3＋1/8）＋（2/5＋1/7）
= 1/2 +(1/3+1/3+1/3) +(1/4+3/4) +(2/5+1/5+2/5) +(5/7+1/7+1/7) +(7/8+1/8)
= 1/2 +1 +1 +1 +1 +1
= 5又1/2