s(n)=(n+1)a(n)/2,
s(n+1)=(n+2)a(n+1)/2,
a(n+1)=s(n+1)-s(n)=[(n+2)a(n+1)-(n+1)a(n)]/2,
(n+1)a(n)=na(n+1),
a(n+1)/(n+1)=a(n)/n=...=a(1)/1=1,
a(n)=na(n+1)/(n+1)=a(n)/n=...=a(1)/1=1,a(n)=n 这步看不懂(n+1)a(n)=na(n+1),等号两边同除以[n(n+1)],a(n)/n=a(n+1)/(n+1).{a(n)/n}是常数列。a(n)/n=a(1)/1=1,a(n)=n