Bạn tham khảo :
$A = 1+2^2+2^3+2^4+2^5+2^6+2^7+2^8+2^9+2^{10}$
$2A = 2 + 2^3 +2^4+2^5+2^6+2^7+2^8+2^9+2^{11}$
$2A -A = ( 2 + 2^3 +2^4+2^5+2^6+2^7+2^8+2^9+2^{11}) - (1+2^2+2^3+2^4+2^5+2^6+2^7+2^8+2^9+2^{10})$
$A = 2 + 2^{11} - 1 $
$A= 2.(1 + 2^{10} )- 1$