两边求倒数,得
1/sn=1/s(n-1) +2
所以数列1/sn是公差为2的等差数列
1/sn=1/s1+(n-1)d
=1+2n-2
1/sn=2n-1
sn=1/(2n-1)
an=sn-s(n-1)
=1/(2n-1)-1/(2n-3)
=-2/[(2n-1)(2n-3)]