等差數(shù)列a1=3,公差=d S2=a1+a2=6+d
bn的公比為q=s2/b2 S2=b3
b2+s2=12
q+q^2=12 各項(xiàng)均為正數(shù) q>0
(q+4)(q-3)=0 q=3
bn=3^(n-1)
Cn=(bn+1/bn)^2
=3^(2n-1)+2+1/3^(2n-1)
{3^(2n-1)}是以3為首項(xiàng),9為公比的等比數(shù)列
{1/3^(2n-1)}是以1/3為首項(xiàng),1/9為公比的等比數(shù)列
{2}是常數(shù)數(shù)列
Tn=3(1-9^n)/(1-9)+2n+1/3(1-(1/9)^n)/(1-1/9)
=3(9^n-1)/8+2n+3(1-1/(9^n))/8