Exact Equations & Substitutions · Easy problem

$(x + 2y)\,dx + (2y - x)\,dy = 0$

$y^2\,dx + xy\,dy = 0$

$(2x + y)\,dx + (x - 3y^2)\,dy = 0$

$3x^2y\,dx + (x^3 + x)\,dy = 0$