第3个回答 2019-06-06
设公差为d,公比为q,依题意
q+q^2=6,q>0,q=2,
8=3d-a1,
a1+5d=16,
解得d=3,a1=1.
(1)an=1+3(n-1)=3n-2.
bn=2^n.
(2)Tn=2+4*2^2+7*2^3+……+(3n-2)*2^n,①
2Tn=............2^2+4*2^3+……+(3n-5)*2^n+(3n-2)*2^(n+1).②
②-①,得Tn=-2-3(2^2+2^3+……+2^n)+(3n-2)*2^(n+1)
=-2-3[2^(n+1)-4]+(3n-2)*2^(n+1)
=10+(3n-5)*2^(n+1).