a) 9.S = 34+ 36+.....+ 31000+ 31002
9.S - S = (34+ 36+.....+ 31000+ 31002) - ( 32+ 34+.....+ 3998+ 31000)
8.S = 31002 - 32
S =31002 - 32 / 8
a) \(S=3^2+3^4+...+3^{998}+3^{1000}\)
\(\Rightarrow3^2.S=3^2.3^2+3^2.3^4+...+3^2.3^{998}+3^2.3^{1000}\)
\(9S=3^4+3^6+...+3^{1000}+3^{1002}\)
\(\Rightarrow8S=9S-S=\left(3^4+3^6+...+3^{1000}+3^{1002}\right)-\left(3^2+3^4+...+3^{998}+3^{1000}\right)\)
\(=3^{1002}-3^2\)
\(=3^{1002}-9\)
\(\Rightarrow S=\dfrac{3^{1002}-9}{8}\)