简便运算1/（2*5）+1/（3*5）+1/(8*11)+……+1/（20*23）=

1/（2*5）+1/（8*5）+1/(8*11)+……+1/（20*23）=
=1/3(1/2-1/5+1/5-1/8+1/8-1/11+...+1/20-1/23)
=1/3(1/2-1/23)
=1/3*21/46
=7/46
你的题目抄错了吧 .