Question

[ngo van hung]

Tính nhanh

7/2+7/4+7/8+7/16+7/32+......+7/512

Answer 1

[T.Ps]

#)Giải :

Đặt \(A=\frac{7}{2}+\frac{7}{4}+\frac{7}{8}+\frac{7}{16}+\frac{7}{32}+...+\frac{7}{512}\)

\(A=\frac{7}{2}+\frac{7}{2^2}+\frac{7}{2^3}+\frac{7}{2^4}+...+\frac{7}{2^9}\)

\(\Rightarrow2A=\frac{7}{2^2}+\frac{7}{2^3}+\frac{7}{2^4}+\frac{7}{2^5}+...+\frac{7}{2^{10}}\)

\(\Rightarrow2A-A=\left(\frac{7}{2^2}+\frac{7}{2^3}+\frac{7}{2^4}+\frac{7}{2^5}+...+\frac{7}{2^{10}}\right)-\left(\frac{7}{2}+\frac{7}{2^2}+\frac{7}{2^3}+\frac{7}{2^4}+...+\frac{7}{2^9}\right)\)

\(\Rightarrow A=\frac{7}{2^{10}}-\frac{7}{2}\)

Answer 2

[ngo van hung]

tiếc nhỉ giải theo kiểu học sinh lớp 4 cơ