Quick Recap — Variables Separable

  • Write as f(y) dy=g(x) dxf(y)\,dy=g(x)\,dx and integrate both sides.
  • dydx=ky⇒y=Cekx\tfrac{dy}{dx}=ky\Rightarrow y=Ce^{kx} (growth/decay).
  • dydx=yx⇒y=Cx\tfrac{dy}{dx}=\tfrac{y}{x}\Rightarrow y=Cx; dydx=xy⇒y2−x2=C\tfrac{dy}{dx}=\tfrac{x}{y}\Rightarrow y^2-x^2=C.
  • Apply initial conditions after integrating to fix CC.