\(\frac{x+2000}{x-2000}=\frac{y+2001}{y-2001}\Rightarrow\left(x+2000\right)\left(y-2001\right)=\left(x-2000\right)\left(y+2001\right)\)
\(\Rightarrow\frac{x+2000}{y+2001}=\frac{x-2000}{y-2001}=\frac{x+2000+x-2000}{y+2001+y-2001}=\frac{2x}{2y}=\frac{x}{y}=\frac{x+2000-\left(x-2000\right)}{y+2001-\left(y-2001\right)}=\frac{2000}{2001}\)
=>đpcm