分类：教育/科学
问题：初中代数求值1/5+1/45+1/117+1/221+1/357 
1/5+1/45+1/117+1/221+1/357=?
回答：1/5+1/45+1/117+1/221+1/357
=1/5+(1/5-1/9)/4+(1/9-1/13)/4+(1/13-1/17)/4+(1/17-1/21)/4
=1/5+1/5/4-1/21/4
=5/21