答：
1/[n(n+4)]=(1/4)*[1/n-1/(n+4)]
所以：
Sn=1/(1*5)+1/(5*9)+...+1/(401*405)
=(1/4)*(1-1/5+1/5-1/9+...+1/401-1/405)
=(1/4)(1-1/405)
=101/405