Question

1｛an｝正项等比数列且a2a4+2a3a5+a4a6=25则a3+a5=___

Answer 1

1 .正项 a2a4=a3a3 ,a4a6=a5a5

a2a4+2a3a5+a4a6=(a3+a5)^2=25 =>a3+a5=5

2.a3=q^2a1,a6=q^2a4

a1(q^2-1)=8,a4(q^2-1)=216 =>a4/a1=27

a4=q^3a1 =>q=3 ,a1=1

同项公式an=3^(n-1)

3.a5=q^3a2 =>162=6q^3 => q=3,a1=2

an=2*3^(n-1)

好多哈~让后来人接着做`

我要睡拉`