Đáp án:
Giải thích các bước giải:
Đặt $A=$$\dfrac{10^{1990}+1}{10^{1991}+1}$
⇒$10A$ $=$$\dfrac{10^{1991}+10}{10^{1991}+1}$
$=$$\dfrac{10^{1991}+1+9}{10^{1991}+1}$
$=1+$$\dfrac{9}{10^{1991}+1}$
Đặt $B=$$\dfrac{10^{1991}+1}{10^{1992}+1}$
⇒$10A$ $=$$\dfrac{10^{1992}+10}{10^{1992}+1}$
$=$$\dfrac{10^{1992}+1+9}{10^{1992}+1}$
$=1+$$\dfrac{9}{10^{1992}+1}$
Ta có: $\dfrac{9}{10^{1991}+1}$>$\dfrac{9}{10^{1992}+1}$
⇒$A>B$