qSn=A2+A3+A4+...A(n+1)
Sn-qSn=A1-A(n+1)=A1(1-q^n)
--->Sn=[A1(1-q^n)]/(1-q)
你也可以到看看http://hi.baidu.com/ephemerasylum/album/item/31354b0efe9416c1aa64575c.html
qSn=A2+A3+A4+...A(n+1)
Sn-qSn=A1-A(n+1)=A1(1-q^n)
--->Sn=[A1(1-q^n)]/(1-q)
你也可以到看看http://hi.baidu.com/ephemerasylum/album/item/31354b0efe9416c1aa64575c.html