\4*5+1\5*6+1\6*7.1\39*40
问题描述：
\4*5+1\5*6+1\6*7.1\39*40
答
太简单了
答
1 \4*5+1\5*6+1\6*7.1\39*40
=1/4-1/5+1/5-1/6+……+1/39-1/40
=1/4-1/40
=9/40