an是等差数列,求​lim (Sn+Sn+1)/(Sn+Sn-1)
lim (Sn+Sn+1)/(Sn+Sn-1)
=[n(n+1)/2+(n+1)(n+2)/2]/[n(n+1)/2+n(n-1)/2]
=(2n²+4n+2)/2n²
=1+2/n+1/n²
我就想知道第一步怎么来的