a, $20,21:0,5+20,21:0,125+20,21×90$
$=20,21:$$\frac{1}{2}+20,21:$ $\frac{1}{8}+20,21×90$
$=20,21×2+20,21×8+20,21×90$
$=20,21×(2+8+90)$
$=20,21×100$
$=2021$
b, $\frac{x}{3}=$ $\frac{14}{x+1}$
$⇒x(x+1)=3×14$
TH1: $x(x+1)=6×7$
$⇒x=6$
TH2: $x(x+1)=-7.(-6)$
$⇒x=-7$
Vậy $x=6$ hoặc $x=-7$
c, $|x-2019|=|x-2020|$
TH1: $x-2019=x-2020$
$⇒0x=-1(loại)$
TH2: $x-2019=2020-x$
$⇒2x=4039$
$⇒x=$$\frac{4039}{2}$
$⇒x=2019,5$
TH3: $x-2020=x-2019$
$⇒0x=1(loại)$
TH4: $x-2020=2019-x$
$⇒2x=4039$
$⇒x=$$\frac{4039}{2}$
$⇒x=2019,5$
Vậy $x=2019,5$
