Bài 1 :Cho S = 1 + 2 + 2^2 +… + 2^2005 Hãy so sánh S với 5.2 ^2004

S=1+2+2²+…+$2^{2005}$ 

=>2S=2+2²+2³+…+$2^{2006}$ 

=>2S-S=(2+2²+2³+….+$2^{2006}$ )-(1+2+2²+…+$2^{2005}$ )

S=$2^{2006}$ -1=2².$2^{2004}$ -1

S=4.$2^{2004}$ -1

vậy S<5.$2^{2004}$ 

@ara boss ris