[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\\
---------\\
\stackrel{octagon}{n=8}
\end{cases}\implies 8\theta =180(8-2)
\\\\\\
\theta =\cfrac{180(8-2)}{8}\implies \theta =\cfrac{45(8-2)}{2}\implies \theta = 135[/tex]
