[tex]\bf \textit{sum of all interior angles in a regular polygon}\\\\ n\theta =180(n-2)~~ \begin{cases} n=\textit{number of sides}\\ \theta =angle~in\\ \qquad degrees\\[-0.5em] \hrulefill\\ n=9 \end{cases}\implies 9\theta =180(9-2) \\\\\\ 9\theta =1260\implies \theta =\cfrac{1260}{9}\implies \theta =140[/tex]
