2A=2+22+23+24+...+211
2A—A=(2+22+23+24+....+211)—(1+2+22+23+...+210)
A=211—1
Ta có A = 2A - A
= \(2\left(1+2+2^2+2^3+...+2^{10}\right)\)- \(\left(1+2+2^2+2^3+....+2^{10}\right)\)
=\(2+2^2+2^3+2^4+.....+2^{11}\)\(-1-2-2^2-2^3-...-2^{10}\)
=\(2^{11}-1\)(Các số còn lại đã trừ hết cho nhau)
\(A=1+2+2^2+2^3+2^4+....+2^{10}\)
\(2A=2+2^2+2^3+2^4+2^5+....+2^{11}\)
\(2A-A=\left(2+2^2+2^3+2^4+2^5....+2^{11}\right)-\left(1+2+2^2+2^3+2^4+...+2^{10}\right)\)
\(A=2^{11}-1\)