\[\boxed{\text{87\ (87).\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\frac{x}{y} = 4\]
\[1)\ \frac{y}{x} = \frac{1}{4}\]
\[2)\ \frac{2x - 3y}{y} = \frac{2x}{y} - \frac{3y}{y} =\]
\[= \frac{2x}{y} - 3 = 2 \cdot \frac{x}{y} - 3 = 2 \cdot 4 - 3 =\]
\[= 8 - 3 = 5\]
\[3)\ \frac{x^{2} + y^{2}}{\text{xy}} = \frac{x^{2}}{\text{xy}} + \frac{y^{2}}{\text{xy}} = \frac{x}{y} + \frac{y}{x} =\]
\[= 4 + \frac{1}{4} = 4\frac{1}{4} = 4,25\]