\[\boxed{\text{493\ (493).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ x^{2} - y^{2} = 0\]
\[(x - y)(x + y) = 0\]
\[y = x;\ \ y = - x\]
\[\textbf{б)}\ \frac{x^{2} - y}{x} = 0\]
\[x^{2} - y = 0;\ \ x \neq 0\]
\[y = x^{2};\ \ x \neq 0\]