Question

Quantas diagonais possui um octogono regular?

Answer 1

D = [n (n - 3)] / 2

n = n° de lados

D = [8 (8 - 3)] / 2
D = [8 × 5] / 2
D = 40 / 2
D = 20

Espero ter ajudado. Valeu!

Answer 2

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