A = 2 + 22 + 23 + ... + 249 + 250
Ta có : 2.A = 2 . ( 2 + 22 + 23 + ... + 249 + 250 )
= 22 + 23 + 24 + ... + 250 + 251
2.A - A = ( 22 + 23 + 24 + ... + 250 + 251 ) - ( 2 + 22 + 23 + ... + 249 + 250 )
A = 251 - 2
Vậy A = 251 - 2
A = 2 + 22 + 23 + ... + 249 + 250
2. A = 22 + 23 + 24 + ... + 249 + 250
2A - A = (22 + 23 + 24 +... + 250 + 251) - (2 + 22 + 23+ 24+...+250)
A = 22 + 23 + 24 +...+ 249 + 251 - 2 - 22 - 23 - 24 - ... 249 - 250
A = (22 - 22) + (23 - 23) + (24 - 24) - (250 - 250) + (251 - 2)
A = 251 - 2
2A=22+23+24+...+250+251
2A-A= 251-(250-250)+...+(22-22)-2
A=251-2
(Nếu tính tổng)
