没有现成的公式,但可以推导出来
S2n=Sn+an+1+an+2+an+3+.+a2n
=Sn+a1q^n+a1q^n+2+a1q^n+3+.+a1q^2n-1
=Sn+q^n(a1+a2+a3+.an)
=Sn+q^nSn