(1) Sn=a1*(1-q^n)/(1-q)=(-a1/(1-q))q^n+(a1/(1-q))
Sn=3^n+a=
比较两式得,
-a1/(1-q)=1,及a1/(1-q)=a
所以:a=-1
(2)将x=an^(1/2),y=a(n-1)^(1/2),代入x-根号2y=0,得:
an^(1/2)-(2a(n-1))^(1/2)=0
an=2a(n-1)
an/a(n-1)=2
所以,an为等比数列
Sn=a1*(1-2^n)/(1-2)=2(2^n-1)=2^(n+1)-2
(1) Sn=a1*(1-q^n)/(1-q)=(-a1/(1-q))q^n+(a1/(1-q))
Sn=3^n+a=
比较两式得,
-a1/(1-q)=1,及a1/(1-q)=a
所以:a=-1
(2)将x=an^(1/2),y=a(n-1)^(1/2),代入x-根号2y=0,得:
an^(1/2)-(2a(n-1))^(1/2)=0
an=2a(n-1)
an/a(n-1)=2
所以,an为等比数列
Sn=a1*(1-2^n)/(1-2)=2(2^n-1)=2^(n+1)-2