In order to calculate the product of a pair of conjugate terms, we can use the pattern below:
[tex](a+b)(a-b)=a^2-b^2[/tex]So we have:
[tex]\begin{gathered} (rs-\frac{2}{5})(rs+\frac{2}{5}) \\ =(rs)^2-(\frac{2}{5})^2 \\ =r^2s^2-\frac{4}{25} \end{gathered}[/tex]