Question

已知数列an和数列bn中

Answer 1

n是等比,则 [2^(a2+1)]=2^(a1+1)x2^(a3+1)

(a2+1)^2=a1+1+a3+1=4

a2=1

an=n-1

bn=2^n

cn=(n-1)2^n

Sn=0x2^1+1x2^2+2x3^ +...+(n-1)2^n

2Sn=0x2^2+1x2^3 +...+(n-2)2^n+(n-1)2^(n+1)

2Sn-Sn=(n-1)2^(n+1)-(2^2+2^3+...+2^n)

Sn=(2n-3)2^n