đặt A=22+(22+23+24+...+220)
đặt P=22+23+24+..+220
=>2P=23+24+25+...+221
=>2P-P=(23+24+25+..+221)-(22+23+24+...+220)
=>P=221-22
=>A=22+221-22=221
vậy A=221
nhớ ****
A=4+(2^2+2^3+2^4+...+2^20)
A-4=(2^2+2^3+2^4+...+2^20)
2(A-4)=2^3+2^4+...+2^21
A-4=2(A-4)-(A-4)=(2^3+2^4+...+2^21)-(2^2+2^3+2^4+...+2^20)
A-4=2^21-4
A=2^21-4+4
A=2^21
Ta có A = 4 + ( 22 + 23 + 24 + .... + 220 )
=> A = 4 + [ ( 23 + 24 + 25 + ..... + 221 ) - ( 22 + 23 + 24 + .... + 220 ) ]
=> A = 4 + 221 - 22
=> A = 22 + 221 - 22
=> A = 221