2/21+2/77+2/166……+2/1677
问题描述：
2/21+2/77+2/166……+2/1677
答
2/21+2/77+2/165+...+2/1677
=2/3*7+2/7*11+2/11*15+...+2/39*43
=(1/3-1/7)/2+(1/7-1/11)/2+...+(1/39-1/43)/2
=(1/3-1/7+1/7-1/11+...1/39-1/43)/2
=(1/3-1/43)/2
=20/129
答
20/129
答
=2/3*7+2/7*11+……+2/39*43
=(1/2)*(1/3-1/7)+(1/2)*(1/7-1/11)+……+(1/2)*(1/39-1/43)
=(1/2)*(1/3-1/7+1/7-1/11+……+1/39-1/43)
=(1/2)*(1/3-1/43)
=20/129