an=-2n+3
当n=1时,
a1=s1=-1+2=1,
当n>=2时,
an=sn-s（n-1）=-n^2+2n-【-(n-1)^2+2(n-1)】=-2n+3;
将a1代入an可知n=1时也满足
故an=-2n+3