ko bt
và đọc nội quy chưa
a) Ta có: \(A=2+2^2+2^3+2^4+...+2^{20}\)
\(2A=2^2+2^3+2^4+2^5+...+2^{21}\)
\(2A-A=2^{21}-2\)
Hay \(A=2^{21}-2\)
a) \(A=2+2^2+2^3+2^4+...+2^{20}\)
\(\Rightarrow2A=2\left(2+2^2+2^3+2^4+...+2^{20}\right)\)
\(\Rightarrow2A=2^2+2^3+2^4+2^5+...+2^{21}\)
\(\Rightarrow2A-A=\left(2^2+2^3+2^4+2^5+...+2^{21}\right)\)
\(-\left(2+2^2+2^3+2^4+...+2^{20}\right)\)
\(\Rightarrow A=2^{21}-2\)
b) Ta có: \(A+2=2^{21}-2+2\)
\(\Rightarrow A=2^{21}=2^{4\cdot5+1}=2^{4\cdot5}\cdot2=\left(...6\right)\cdot2=\left(...2\right)\)
Vậy A không phải là số chính phương (đpcm)