Question

quantas diagonais tem um decágono?

Answer 1

o decagono tem 10 lados, entao aplicando a formula:
d=(10(10-3)/2)=70/2=35, ou seja o decagono tem 35 diagonais. 

Answer 2

$\large\boxed{\begin{array}{l} \rm{d = \dfrac{n \cdot(n - 3)}{2}} \\ \\ \rm{d = \dfrac{10 \cdot(10 - 3)}{2} } \\ \\ \sf{d = \dfrac{10 \cdot7}{2} } \\ \\ \rm{d = \dfrac{70}{2} } \\ \\\huge\boxed{\boxed{\boxed{\boxed{\rm{d = 35}}}}} \end{array}}$