等比数列错位相减法怎么做

Question

如题所述

Answer 1

Sn=a1+ a1q+ a1q^2+ a1q^3+ …+a1q^n ………. (1)
qSn=a1q+ a1q^2+ a1q^3+ a1q^4…+a1q^n+ a1q^(n+1) …….. (2)
(1)-(2)
得（1-q）Sn=a1-a1q^(n+1)
q不等于1时 Sn=a1[1-q^(n+1)]/(1-q)
q=1时 Sn=n a1追问