Question

qual o número de diagonais de um octógono?

Answer 1

20 diagonais

P = n (n-3) /2

P = 8 (8-3) /2

P = 8* 5 /2

P = 40 / 2

P = 20 diagonais

Answer 2

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