Cách 1:
$\begin{align}
& \frac{1999\times 2001-1}{1998+1999\times 2000}\times \frac{7}{5}=\frac{1999\times (2000+1)-1}{1998+1999\times 2000}\times \frac{7}{5} \\
& =\frac{1999\times 2000+1999-1}{1998+1999\times 2000}\times \frac{7}{5}=\frac{1999\times 2000+1998}{1998+1999\times 2000}\times \frac{7}{5}=\frac{7}{5} \\
\end{align}$
Cách 2:
$\begin{align}
& \frac{1999\times 2001-1}{1998+1999\times 2000}\times \frac{7}{5}=\frac{1999\times 2001-1}{1999-1+1999\times 2000}\times \frac{7}{5} \\
& =\frac{1999\times 2001-1}{1999\times (1+2000)-1}\times \frac{7}{5}=\frac{1999\times 2001-1}{1999\times 2001-1}\times \frac{7}{5}=\frac{7}{5} \\
\end{align}$