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