Question

tinh tong S=1+3+3²+....+3¹⁸+3¹⁹

can gap

Answer 1

S=1+3+3^2+....+3^18+3^19

3S=3+3^2+3^3+...+3^19+3^20

3S-S=3+3^2+3^3+...+3^19+3^20-1-3-3^2-...-3^18-3^19

2S=3^20-1

S=(3^20-1):2

Answer 2

ta có

3S=3+3²+3³+..........................+3¹⁹+3²⁰

3S-S=3²⁰-1

2S=3²⁰-1

S=3²⁰-1/2

Answer 3

<=> 3S = 3.( 1 + 3 + 3² + .. + 3¹⁸ + 3¹⁹ )

<=> 3S = 3 + 3² + 3³ + 3⁴ + ... + 3¹⁹ + 3²⁰

=> 3S - S = 2S <=> ( 3 + 3² + 3³ + 3⁴ + ... +3¹⁹ + 3²⁰ ) - ( 1 + 3 + 3² + 3³ + ... + 3¹⁸ + 3¹⁹)

=> 2S = 3²⁰ - 1

=> S = ( 3^{20 - 1} ) : 2

Vậy S = ( 3²⁰ - 1 ) : 2