先求an表达式,
n边形,扩展开分成每边看,每一边先被分成3段,左右2段,中间1段去掉,加n-1边,原来每边变成（n-1）+2=n+1边,共n边
an= （n+1）n
a5=5*6=30
1/an=1/（n+1）n=1/n-1/(n+1)
1/a3+1/a4+1/a5+…+1/an
=1/3-1/4+1/4-1/5+1/5-1/6+…+1/(n-1)-1/n+1/n-1/(n+1)
=1/3-1/(n+1)=197/600
n=199
先求an表达式,
n边形,扩展开分成每边看,每一边先被分成3段,左右2段,中间1段去掉,加n-1边,原来每边变成（n-1）+2=n+1边,共n边
an= （n+1）n
a5=5*6=30
1/an=1/（n+1）n=1/n-1/(n+1)
1/a3+1/a4+1/a5+…+1/an
=1/3-1/4+1/4-1/5+1/5-1/6+…+1/(n-1)-1/n+1/n-1/(n+1)
=1/3-1/(n+1)=197/600
n=199