libertinus-test-equations.tex

\documentclass{scrbook}  
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\begin{document}

\begin{align*}
\left (x+\alpha \right )^{n}&=\sum _{k=0}^{n} \binom{n}{k} x^{k}\alpha ^{n-k} \\
\left |x\right |&=\begin{cases} -x,\quad & x<0\\
		  x, &x\geq 0
                  \end{cases}
\\
\nabla \cdot \nabla \psi
&=\frac{\partial ^{2}\psi }{\partial x^{2}}+\frac{\partial ^{2}\psi }{\partial y^{2}}+\frac{\partial ^{2}\psi }{\partial z^{2}} \\
&=\frac{1}{r^{2}\sin ⁡\theta }
\left [\sin ⁡\theta \frac{\partial }{\partial r}\left (r^{2}\frac{\partial \psi }{\partial r}\right ) \right. \\
  &\qquad \left. +\frac{\partial }{\partial \theta }\left (\sin ⁡\theta \frac{\partial \psi }{\partial \theta }\right )
  +\frac{1}{\sin ⁡\theta }\frac{\partial ^{2}\psi }{\partial \varphi ^{2}}\right ] 
\end{align*}

\end{document}