Question

Quantas diagonais tem um octógono​

Answer 1

Octógono = 8 lados.

diagonais = (n-3).n / 2

diagonais = (8-3).8/2

diagonais = 5. 4

diagonais = 20

Answer 2

$\large\displaystyle\boxed{\begin{array}{l} \rm{d = \dfrac{n \cdot(n - 3)}{2} \rightarrow \begin{cases} \rm \: d = diagonais \\ \rm \: n = n\acute{u}mero\,de\,lados \end{cases} } \\ \rm{d = \dfrac{8 \cdot(8 - 3)}{2} } \\ \\ \rm{d = \dfrac{8 \cdot5}{2} } \\ \\ \rm{d = \dfrac{40}{2} } \\ \\ \boxed{ \boxed{ \rm{d = 20}}} \checkmark\end{array}}$