Therefore, {1/sn} is the arithmetic progression with the first term1/s1=1/a1= 3 and the tolerance is 2.

So1/sn = 3+2 (n-1) = 2n+1.

So sn= 1/(2n+ 1)

Then an = sn-s (n-1) =-2/(4N2-1).

Therefore, Lima/sn 2 =-2.

You can also calculate that {1/sn} is the arithmetic progression with the first term of1/s1=1/a1= 3 and the tolerance is 2.

Get limsn=lims(n- 1)。

An=-2SnS(n- 1) can be divided by SnS(n- 1) at the same time.

Li Man /Sn 2 =-2 can also be obtained.