Question

Tìm x biết

x*(1/2+1/4+1/8+...+1/512)=511/256

Answer 1

A= 1/2 + 1/4+ 1/8+ 1/16 + 1/32 + 1/64 + 1/128 + 1/256 + 1/512 (1) 

=> 2A=1+ 1/2 + 1/4+ 1/8+ 1/16 + 1/32 + 1/64 + 1/128 + 1/256 (2) 

Lấy (1)-(2) 
=> 2A-A=(1+ 1/2 + 1/4+ 1/8+ 1/16 + 1/32 + 1/64 + 1/128 + 1/256)-(1/2 + 1/4+ 1/8+ 1/16 + 1/32 + 1/64 + 1/128 + 1/256 + 1/512) 

=>A=1-1/512=511/512 

5* nha bạn

Answer 2

cảm ơn bạn nhiều nha.Mình rồi đó