.设An是等比数列a3=1.a6=81.求An的通项公式An2.等差数列Bn中,b1=-2.b3=a4+a5求数列Bn的前n项和公式Sn过程写出来..

Question

.设An是等比数列a3=1.a6=8
1.求An的通项公式An 2.等差数列Bn中,b1=-2.b3=a4+a5求数列Bn的前n项和公式Sn 过程写出来..

Answer 1

一楼算错了!
1.a6 = a3*q^3, q = 2
An = a3*q^(n-3) = 2^(n-3)
2.a4 = 2,a5 = 4,b3 = a4+a5=6
b3 = b1+ 2d,公差=（6+2）/2 = 4,bn = b1+(n-1)d = 4n-6
Sn = (b1+bn)n/2 = (4n-6-2)n/2 = 2n(n-2)