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The sum of a and b is
\( a + b\)  

The sum of $a$ and $b$ is $ a + b $

\begin{math}
 The sum of a and b is a + b
\end{math}
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The Pythagorean Theorem can be stated as $c^{2}=a^{2}+b^{2}$. Its named after a Greek mathematician. \\

The Pythagorean Theorem can be stated as \[c^{2}=a^{2}+b^{2}\] Its named after a Greek mathematician.

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{\huge $ x_{ij}^{\mu}$}

\begin{displaymath}
\lim_{n \to \infty}
\sum_{k=1}^n 
\frac{1}{k^2}
\end{displaymath}
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\begin{equation} \label{eq_1}
\epsilon> 0
\end{equation}

\begin{equation} \label{eq_2}
\Delta> 0
\end{equation}

Eqn (\ref{eq_1}) $\neq$ Eqn (\ref{eq_2})
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