问题标题：
(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+……(1/19+1/20)
问题描述：
(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+……(1/19+1/20)
宋国文回答：
(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+……(1/19-1/20)
=1/2-1/3+1/3-1/4+1/4-1/5+1/5-……-1/19+1/19-1/20
=1/2-1/20
=9/20
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