\[25x² + xy + 4y^{2}\]
\[25x^{2} + 20xy + 4y^{2} = (5x + 2y)^{2}\]
\[\frac{1}{16}x^{2} + xy + 4y^{2} = \left( \frac{1}{4}x + 2y \right)^{2}\]
\[25x^{2} + xy + \frac{1}{100}y^{2} =\]
\[= \left( 5x + \frac{1}{10}y \right)^{2}.\]