Problem 02
$y(y^3 - x) \, dx + x(y^3 + x) \, dy = 0$
Solution 02
$y(y^3 - x) \, dx + x(y^3 + x) \, dy = 0$
$y^4 \, dx - xy \, dx + xy^3 \, dy + x^2 \, dy = 0$
$(y^4 \, dx + xy^3 \, dy) - (xy \, dx - x^2 \, dy) = 0$
$y^3(y \, dx + x \, dy) - x(y \, dx - x \, dy) = 0$
$y^3 \, d(xy) - x(y \, dx - x \, dy) = 0$
Divide by y3
$d(xy) - \dfrac{x}{y} \left( \dfrac{y \, dx - x \, dy}{y^2} \right) = 0$
$d(xy) - \dfrac{x}{y} \, d\left( \dfrac{x}{y} \right) = 0$