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\begin{document}

\begin{table}[h]
\centering\begin{tabular}{|c||c|c|c|c|c|c|}
\hline
\backslashbox{$\theta_1$}{$\theta_2$}&\ang{0}&\ang{30}&\ang{45}&\ang{60}&\ang{90}&\ang{120}\\
\hline 
\ang{0} &$-1$&$-\frac{\sqrt 3}{2}$&$-\frac1{\sqrt 2}$&$\frac12$&0&$\frac12$\\
\hline
\end{tabular}
\end{table}

\end{document}
