(i) A convex quadrilateral

Convex quadrilateral has two diagonals.

(ii) A regular hexagon

An n-sided convex polygon has diagonals.

∴Applying the above relation for hexagon:

Number of diagonals =

Regular Hexagon has 9 diagonals.

(iii) A triangle

An n-sided convex polygon has diagonals.

∴Applying the above relation for triangle:

Number of diagonals =

Therefore a triangle has no diagonal