$\phantom{X}d_n\,=\,\dfrac{\;n(n\,-\,3)\;}{2}\;=\;20\phantom{X} \;\Longleftrightarrow\;$ $\;n^2 - 3n - 40 = 0\;\Longleftrightarrow\;$ $\,\left\{\begin{array}{rcr} n\,=\,8\phantom{XX}\longleftarrow & {\text (resposta)\phantom{X}} \\ n\,=\,\cancel{-5}\;\longleftarrow& {\text (inadequado)} \\ \end{array} \right.\,$
se n = 8 o polígono é o OCTÓGONO