A=4+22 +23+24+....220
A=22+22 +23 +24 +....220
2A=2(4+22 +23 +24 +....220)
2A=23+23 +24 +25 +....221
2A-A=(23+23 +24 +25 +....221)-(22+22 +23 +24 +....220)
A=23+221-(22+22)
A=8+221 - 8
A=221
Vậy A= 221
2A=2.(4 + 22 + 23 + 24 + ... + 220 )
2A=8+2^3+....+2^21
2A-A=(8+2^3+....+2^21)-(4+2^2+....+2^20)
A=8-(4+2^2)+2^21
A=2^21
A = 4+ ( 22 + 23 + 24 +... + 220)
A-4 = 22 + 23+ 24 +... + 220
2( A-4) = 23 + 24 + 25 +.. + 221
A-4= 2(A-4) -(A-4) =(23 + 24 + 25 +.. + 221) - ( 22 + 23 + 24 +... + 220)
A-4 = ( 23-23) + ( 24-24) + 25- 25) + ....+(221 - 221)
A-4 = 221 -4
A = 221 -4 + 4
A= 221
Tính: A = 4 + 22 + 23 + 24 + ... + 219 + 220 (1)
Ta co : 2.A = 8 + 23 + 24 + 25 + ... + 220 + 221 (2)
Lầy (2) trư (1) dược: A = 2.A - A = 221
