Math — Partial Derivatives

\frac{\partial f}{\partial x}, \quad \frac{\partial f}{\partial y}

\nabla f = \left(\frac{\partial f}{\partial x},\ \frac{\partial f}{\partial y}\right)

df = \frac{\partial f}{\partial x}dx + \frac{\partial f}{\partial y}dy

\text{critical point: } \nabla f = \mathbf{0}