(1)A = \pi \cdot r^2

A = π \cdot r^{2}

(2)C = 2\pi r

C = 2 π r

(3)x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}

x = \frac{- b \pm \sqrt{b^{2} - 4 a c}}{2 a}

(4)

\frac{\mathrm{D} \mathbf{u}}{\mathrm{D} t} =
\frac 1 \rho \nabla \cdot \boldsymbol{\sigma} + \mathbf{f}.

\frac{D u}{D t} = \frac{1}{ρ} \nabla \cdot σ + f .