解
原式
=(1/3-1/4)+(1/4-1/5)+(1/5-1/6)+……+(1/19-1/20)
=1/3+(1/4-1/4)+(1/5-1/5)+……+(1/19-1/19)-1/20——内部抵消
=1/3-1/20
=20/60-3/60
=17/60