数列1请看mhc286附件
(1)。 S(n+1)-Sn = An +1 = 2Sn +1
==> [S(n+1) +1/2]/[Sn +1/2] = 3
==> Sn +1/2 =(S1 +1/2)*3^(n-1) ==> Sn=(3^n -1)/2
An = Sn -S(n-1) = 3^(n-1)
(2)。 A1=1,A2=3,A3=9, B1=B2-d,B2,B2+d
T3 = (B2-d)+B2+(B2+d)=15 ==> B2=5
B2^2 = (B2-d)(B2+d) ==> d=2, -10(舍去)
==> Tn = 2n +1
(3)。 Am=a*m+m(m-1)d/2, An=a*n+n(...全部
(1)。 S(n+1)-Sn = An +1 = 2Sn +1
==> [S(n+1) +1/2]/[Sn +1/2] = 3
==> Sn +1/2 =(S1 +1/2)*3^(n-1) ==> Sn=(3^n -1)/2
An = Sn -S(n-1) = 3^(n-1)
(2)。
A1=1,A2=3,A3=9, B1=B2-d,B2,B2+d
T3 = (B2-d)+B2+(B2+d)=15 ==> B2=5
B2^2 = (B2-d)(B2+d) ==> d=2, -10(舍去)
==> Tn = 2n +1
(3)。
Am=a*m+m(m-1)d/2, An=a*n+n(n-1)d/2
m^2/n^2 =Sm/Sn =[a*m+m(m-1)d]/2/[a*n+n(n-1)d/2]
==> (m-n)(2a-d)=0 ==> 2a=d
==> am/an =[a+(m-1)d]/[a+(n-1)d]
= (2m-1)/(2n-1)
。
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