Question

Answer 1

\(A=\dfrac{1}{4}+\dfrac{1}{8}+...+\dfrac{1}{512}\\ \Rightarrow2A=\dfrac{1}{2}+\dfrac{1}{4}+...+\dfrac{1}{256}\\ \Rightarrow2A-A=\left(\dfrac{1}{2}+\dfrac{1}{4}+...+\dfrac{1}{256}\right)-\left(\dfrac{1}{4}+\dfrac{1}{8}+...+\dfrac{1}{512}\right)\\ \Rightarrow A=\dfrac{1}{2}+\dfrac{1}{4}+...+\dfrac{1}{256}-\dfrac{1}{4}-\dfrac{1}{8}-..-\dfrac{1}{512}\\ \Rightarrow A=\dfrac{1}{2}-\dfrac{1}{512}\\ \Rightarrow A=\dfrac{255}{512}\)

`->D`

Answer 2

(D) 255/512

Answer 3

ủa? Sao mệnh danh là toán 5 mà lúc ôn toán 4 cấp QG cô mik đã cho học dạng này rùi ?