a1+a2+a3+……+an＝n^2an
a1+a2+a3+……+a(n-1)＝(n-1)^2a(n-1)
两式相减得
an＝n^2an-(n-1)^2a(n-1)
(n^2-1)an=(n-1)^2a(n-1)
an/a(n-1)=(n-1)^2/(n^2-1)
an/a(n-1)=(n-1)^2/[(n-1)(n+1)]
an/a(n-1)=(n-1)/(n+1)
an/a(n-1)=(n-1)/(n+1)
.
a3/a2=2/4
a2/a1=1/3
以上等式相乘得
an/a1=2/[n(n+1)]
an/(1/2)=2*[1/n-1/n(n+1)]
an=1/n-1/(n+1)
s60=1-1/2+1/2-1/3+.+1/60-1/61)
=1-1/61
=60/61