Ta có :
A = 4 + 22 + 23 + 24 +. . . + 220 ( 1 )
2A = 8 + 23 + 24 + 25 + . . . + 221 ( 2 )
Lấy ( 2 ) - ( 1 ), ta có
2A - A = ( 8 + 23 + 24 + 25 + . . . + 221 ) - ( 4 + 22 + 23 + 24 +. . . + 220 )
A = 8 + 221 - 4 + 22
A = 8 + 221 - 8
A = 221
Ta có: A = 4 + 22 + 23 + 24 +. . . + 220
=> 2A = 8 + 23 + 24 + . . . + 221.
=> 2A – A = 221 +8 – ( 4 + 22 ) + (23 – 23) +. . . + (220 – 220). = 221.
A=4+22+23+24+...+220
Đặt B=22+23+24+...+220
2B=23+24+25+...+221
B=2B-B=221-22
\(\Rightarrow\)A=4+(221-22)
\(\Rightarrow\)A=22+221-22
\(\Rightarrow\)A=221