微分方程 differential equation Ex: dydx=f(x)\frac{dy}{dx} = f(x)dxdy​=f(x) y=∫f(x)dxy= \int f(x) dxy=∫f(x)dx solved substitution Ex2: (ddx+x)(\frac{d}{dx}+x)(dxd​+x)为annihilation operator 湮没算符 in quantum mechnics dydx=−xy\frac{dy}{dx} = -xydxdy​=−xy dyy=