S = 1 +2 + 2 ² + 2 ³ + ............ + `2^9`
2S = 2 + 2 ² + 2 ³ + ............ + `2^10`
2S -S = (2 + 2 ² + 2 ³ + ............ + `2^10`) - (1 +2 + 2 ² + 2 ³ + ............ + `2^9`)
S = `2^10` - 1
Ta có 5 · `2^8` = (`2^2` + 1) . `2^8` = `2^2` . `2^8` + `2^8` = `2^10` + `2^8`
mà `2^10` + `2^8` > `2^10` - 1
=> 5 · `2^8` > S
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