\[\boxed{\text{921\ (921).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ \sqrt{50x} + \sqrt{32x} - \sqrt{98x} =\]
\[= 5\sqrt{2x} + 4\sqrt{2x} - 7\sqrt{2x} = 2\sqrt{2x};\]
\[= a - 2 - a + \sqrt{2a} = \sqrt{2a} - 2;\]
\[\textbf{в)}\ \left( \sqrt{x} + \sqrt{y} \right)^{2} - \left( \sqrt{x} - \sqrt{y} \right)^{2} =\]
\[= x + 2\sqrt{\text{xy}} + y - x + 2\sqrt{\text{xy}} - y =\]
\[= 4\sqrt{\text{xy}};\]
\[\textbf{г)}\ \left( \sqrt{x} - \sqrt{y} \right)\left( x + \sqrt{\text{xy}} + y \right) =\]
\[= x\sqrt{x} - y\sqrt{y}.\]