[tex]\bf \textit{sum of interior angles of a polygon}\\\\
n\theta=180(n-2)\qquad
\begin{cases}
n=\textit{number of sides}\\
\theta=\textit{internal angle}\\
--------------\\
n=20
\end{cases}
\\\\\\
thus\implies 20\cdot \theta=180(20-2)\implies 20\cdot \theta=3240
\\\\\\
\theta=\cfrac{3240}{20}[/tex]
