a ) S =3¹+3²+....+3 mũ 2019 +3 mũ 2020
=>3S = 3 . ( 3¹+3²+....+3 mũ 2019 +3 mũ 2020 )
=> 3S = 3²+....+3 mũ 2019 +3 mũ 2021
=> 3S - S =( 3²+....+3 mũ 2019 +3 mũ 2021 ) - ( 3¹+3²+....+3 mũ 2019 +3 mũ 2020 )
=> 2S = 3 mũ 2021 - 3¹
=> S = ( 3 mũ 2021 - 3¹ ) : 2
b ) S = (3¹+3²)+....+(3 mũ 2019) +(3 mũ 2020)
=>S = 3 .(1+3)+3³.(1+3)+...+3 mũ 2009.( 1 + 3 )
=>S = 3 . 4 + 3³ . 4 + 3³ . 4
=>S = 4 . (3+ 3³+...+3 mũ 2009 ) chia hết cho 4
=> S chia hết cho 4
S=(3¹+3²)+....+(3 mũ 2019) +(3 mũ 2020)
=>S= 3¹.(1+3²) + 3³.(1.3² ) + ...+3 mũ 2019.(1+3²)
=>S= 3¹. 10 + 3³. 10 +...+3 mũ 2019 . 10
=>S=10 . (3¹+3³+...+3 mũ 2019) chia hết cho 10
=>S chia hết cho 10
