30min内回答分数双倍.已知等差数列{an}满足公差d=2,a6=12,等比数列{bn}满足b1=a1,前三项和为6,（1）求{an}前6项和（2）求bn

Question

30min内回答分数双倍.
已知等差数列{an}满足公差d=2,a6=12,等比数列{bn}满足b1=a1,前三项和为6,
（1）求{an}前6项和
（2）求bn

Answer 1

(1)
a6=12=a1+5d=a1+10
a1=2
an前六项和=(2+12)*6/2=42
(2)设q是公比
b1=2,前三项和=b1+b2+b3=b1+b1*q+b1*q^2=2(1+q+q^2)=6
q^2+q+1=3
(q-1)(q+2)=0 q=1或者q=-2
所以bn=2或者bn=2*（-2）^(n-1)