Đáp án:
$A=$$\frac{1}{2}$ + $\frac{1}{2²}$ + $\frac{1}{2³}$ +....+ $\frac{1}{ 2^{100} }$
$2A= 1 +$ $\frac{1}{2}$ + $\frac{1}{2²}$ + ......+ $\frac{1}{ 2^{99} }$
$2A-A=1-$$\frac{1}{ 2^{100} }$
$A =$ $\frac{ 2^{100} }{ 2^{100} }$ - $\frac{1}{ 2^{100} }$
$A =$ $\frac{2^{100}-1}{ 2^{100} }$
$⇒$ $\frac{2^{100}-1}{ 2^{100} }$ $< 1 ( đpcm)$