A = 20+21+22+23+24
2A = 21+22+23+24+25
2A - A = 25 - 20
=> A = 25 - 1 = B
=> A = B
A=20+21+22+23+24
=>2A=21+22+23+24+25
=>2A-A=(21+22+23+24+25)-(20+21+22+23+24)
=>A=25-1
=>A=B
Vậy A=B
A = 20 + 21 + 22 + 23 + 24
=> 2A = 2 + 22 + 23 + 24 + 25
=> 2A - A = (2 + 22 + 23 + 24 + 25) - (20 + 21 + 22 + 23 + 24)
=> A = 25 - 1 =B