画圈题具体步骤高二数列 画圈题具体步骤
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画圈题具体步骤高二数列画圈题具体步骤画圈题具体步骤高二数列 画圈题具体步骤画圈题具体步骤高二数列画圈题具体步骤(3)a3=2,a7=1letbn=1/(an+1)bn=b1+(n-1)db7
画圈题具体步骤高二数列 画圈题具体步骤
画圈题具体步骤
高二数列 画圈题具体步骤
画圈题具体步骤高二数列 画圈题具体步骤
(3)
a3=2,a7=1
let
bn = 1/(an+1)
bn = b1+(n-1)d
b7-b3 = 4d
1/(a7+1) - 1/(a3+1) = 4d
1/2-1/3 =4d
d= 1/24
b3 = b1+2d
1/3 = b1+1/12
b1= 1/4
1/(an+1) - 1/(a(n-1)+1) = d
=1/24
1/(an+1) - 1/(a1+1) = (n-1)/24
1/(an+1) - 1/4 =(n-1)/24
1/(an+1) =(n+5)/24
an+1 = 24/(n+5)
an = -1 + 24/(n+5)
(5)
an =a1+(n-1)d
a1=1/3
a2+a5 =4
2a1+ 5d =4
2/3+5d = 4
d= 2/3
an = a1+(n-1)d = 33
1/3 + (n-1)(2/3) =33
1+2(n-1)=99
n=50
(6)
an =a1+(n-1)d ( d>0)
a1+a2+a3=15
3a1+3d =15
a1+d=5 (1)
a1a2a3=80
a1(a1+d)(a1+2d) =80
a1(5)(10-a1)=80
10a1-(a1)^2=16
(a1)^2 - 10a1+16=0
(a1-2)(a1-8)=0
a1=2 or 8 (rejected)
a1=2 => d=3
a11+a12+a13= 3a1+ 33d = 3(2) + 33(3) = 105
好着急啊，这么简单都不会
6题 用a1 和 xq 分别表示a1 a2 a3 解出a1 和q在带到后面的方程中可得答案