Question

5- Calcule o número de diagonais de um octógono.
a) 20
b) 30
c) 40
d) 50​

Answer 1

$\large\boxed{\begin{array}{l} \rm \: d = \dfrac{n \: . \: (n - 3)}{2} \\ \\ \rm \: d = \dfrac{8 \: . \:(8 - 3) }{2} \\ \\ \rm \: d = \dfrac{8 \: . \: 5}{2} \\ \\ \rm \: d = \dfrac{40}{2} \\ \\ \boxed{ \boxed{ \rm{d = 20}}}\leftarrow\textsf{Letra A}\end{array}}$

Answer 2

$\large{\begin{array}{l}\tt d=\dfrac{n\cdot(n-3)}{2}\\\\\tt d=\dfrac{8\cdot(8-3)}{2}\\\\\tt d=\dfrac{8\cdot5}{2}\\\\\tt d=\dfrac{40}{2}\\\\\boxed{\boxed{\tt d=20}}\end{array}}$