Question

quantas diagonais possue o decágono?

Answer 1

n = 10
D = ?
-------------------

D = n(n - 3) 
         2

D = 10.(10 - 3)
            2

D = 10 . 7
         2

D = 70/2

D = 35

Answer 2

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