Question

tinh         4+2^2+2^3+2^4+...+2^20

Answer 1

Đặt A = 4 + 2^2 + 2^3 + ... + 2^20

=> A = 4 + ( 2^2 + 2^3 + .... + 2^20 )

=> A = 4 + [ ( 2^3 + 2^4 + ... + 2^21 ) - ( 2^2 + 2^3 + .... + 2^20 ) ]

=> A = 4 + ( 2^21 - 2^2 )

=> 2^2 + 2^21 - 2^2

=> A = 2^21

Answer 2

Đặt A=4+2^2+2^3+2^4+...+2^20

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

2A=2^3+2^3+2^4+...+2^20+2^21

2A-A=(2^21+2^20+...+2^4+2^3+2^3)-(2^20+2^19+...+2^4+2^3+2^2+2^2)

A=2^21+2^20+2^19+2^18+...+2^4+2^3+2^3-2^20-2^19-...-2^4-2^3-2^2-2^2

A=2^21+2^3-2^2

A=2^21+(8-4)

A=2^21-4