(1)由题意,可得
an=(1/4)^n;
那么:
bn+2=3*log(1/4)an=3n;
所以:
bn=3n-2,为等差数列;
(2)由条件Cn= an*bn得到:
Cn= (1/4)^n*(3n-2)=3n*(1/4)^n-2*(1/4)^n
记Cn的前n项和为Sn;那么:
Sn=3[1/4+2*(1/4)^2+……+n*(1/4)^n]-2*(1/4+(1/4)^2+……+(1/4)^n);
记Pn=1/4+2*(1/4)^2+……+n*(1/4)^n; --------(1)
则有:
1/4*Pn=(1/4)^2+2*(1/4)^3+……+n*(1/4)^(n+1); ------(2)
(1)-(2)得到:
3/4 Pn=1/4+(1/4)^2+(1/4)^3+……+(1/4)^n-n*(1/4)^(n+1)
= 1/3*(1-(1/4)^n)- n*(1/4)^(n+1)
所以Sn可变形为:
Sn=3[1/3*(1-(1/4)^n)- n*(1/4)^(n+1)]-2*[1/3*(1-(1/4)^n)]
=1/3*[1-(1/4)^n]-3n*(1/4)^(n+1);