设通项公式 an=a1+(n-1)d ∴ Sn=na1+n(n-1)d/2 45=n+n(n-1)*4/2 45=n+2n(n-1) 45=n+2n^2-2n 2n^2-n-45=0 (2n+9)(n-5)=0 n=5 ∴an=a1+(n-1)d =1+4(5-1) =17
