### Theory:

We can define a quadrilateral in two ways:
• A quadrilateral is a closed-form that is obtained by joining four points which are non-collinear to any three points.

• A quadrilateral is a shape which is closed with four vertices and four sides.

We can say that there are four sides, four angles, and four vertices are in a quadrilateral.

Think of the following quadrilateral, for example:

A quadrilateral $$ABCD$$ will have two diagonals: $$AD$$ and $$BC$$.

• They have $$4$$ sides.
• They have $$4$$ vertices.
• They have $$2$$ diagonals.
• The sum of all interior angles is $$360°$$.