可能的情况
1,3,6,10,...
+2,+3,+4 =>a(n+1)=a(n)+n+1
a(n)=[n(n+1)]/2
1,4,9,16,...,
1^2,2^2,3^2,4^2,=>b(n)=n^2
n^2=[n(n+1)]/2
n=1