Question

1/2 +1/4+1/8+..+1/256 

Làm 2 cách giúp mk mha

Answer 1

Đặt A=1/2+1/4+1/8+...+1/256

2A=2/4+2/8+2/16+...+2/512

2A—A=(2/4+2/8+2/16+...+2/512—1/2+1/4+1/8+...+1/256)

A=2/512—1/2

Answer 2

C1 :Đặt B=1/2+1/4+..+1/256

=> 2B=1+1/2+...+1/128

=> 2B-B=(1+1/2+...+1/128)-(1/2+1/4+...+1/256)

=> B=1-1/256

=>. B=255/256

Vậy 1/2+1/4+..+1/256=255/256