1.设这2个数是x,y
6,x,y等差数列:2x=y+6
x,y,16成等比数列:y²=16x
解得:{x=1 y=-4 或 {x=9 y=12
2.a1+a(2n-1)=2an
2008=[a1+a(2n-1)](2n-1)/2=an(2n-1)=8(2n-1)=2008
n=126
3.a(n+1)=a1an
a2=a1·a1
(a1)²=9
a1=3
a(n+1)/an=a1=3
这是等比数列,首项为3,公比为3
数列{an}的通项公式是:an=3^n
4.F=P*(1+i)^n
100000×(1+5.7%)^10-100000≈74080(元)
5.a7+a9=a4+a12
a12=a7+a9-a4=16-1=15
6.(a3)²=a2·a6
(a2+d)²=a2·(a2+4d)
(a2)²+2a2d+d²=(a2)²+4a2d
d²=2a2d
d≠0
d=2a2
a3=a2+d=3a2
公比为a3/a2=3a2/a2=3
7.an=2^n
lgan=lg2^n=nlg2