(1)an=21-2n
Sn=(a1+an)*n/2=(20-n)n=-n^2+20n
(2)bn-an=3^(n-1)
bn=3^(n-1)+an=3^(n-1)+21-2n
Tn=(1+3+9+...+3^(n-1))+21n-n(n+1)
=(3^n-1)/2+21n-n^2-n
=3^n /2 -n^2 +20n -1/2
(1)an=21-2n
Sn=(a1+an)*n/2=(20-n)n=-n^2+20n
(2)bn-an=3^(n-1)
bn=3^(n-1)+an=3^(n-1)+21-2n
Tn=(1+3+9+...+3^(n-1))+21n-n(n+1)
=(3^n-1)/2+21n-n^2-n
=3^n /2 -n^2 +20n -1/2