Question

Tinh

A=1+3+3^2+3^3+......+3^19+3^20

Answer 1

A=1+3+3²+3³+...+3¹⁹+3²⁰

3.A=3.(1+3+3²+3³+...+3¹⁹+3²⁰)

3.A=3+3²+3³+3⁴+...+3²⁰+3²¹

3.A-A=(3+3²+3³+3⁴+...+3²⁰+3²¹)-(1+3+3²+3³+...+3¹⁹+3²⁰)

2.A=3²¹-1

A=(3²¹-1):2

Answer 2

bằng (3²¹-1):2 hoặc 5230176601

Answer 3

A=1+3+3^2+3^3+......+3^19+3^20

3A= 3 . ( 1+3+3^2+3^3+......+3^19+3^20 )

3A = 3+3^2+3^3+......+3^19+3^20+3^21

3A - A = ( 3+3^2+3^3+......+3^19+3^20+3^21 )

- ( 1+3+3^2+3^3+......+3^19+3^20 )

2A = 3^21 - 1

=> A = (3^21 - 1) : 2