Triangle Diagonals

A triangle has zero diagonals.

Diagonals are created across vertices in a polygon, but the vertices must not be adjacent to one another.

A triangle has only adjacent vertices. Hence triangle has zero diagonals.

$\displaystyle \frac { n ( n - 3 ) } { n } \Rightarrow \frac { 3 \times 0 } { 3 } = 0$ .

Thus, a triangle has no diagonals.