Question

calcular o numero de diagonais de um octógono

Answer 1

d = n(n-3)/2, onde n = 8 (octógono)
d = 8.(8-3)/2
d = 8.5/2
d = 40/2
d = 20

Answer 2

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