# Quadrilateral-Basic Notions

- Author:
- Jorge Cássio

- Topic:
- Angles, Quadrilaterals

## Quadrilateral - Definition

Given four distinct points A, B, C and D of the same plane, neither three points are collinear ones. If segments AB, BC, CD and AD intersect only at the ends, the joining of these segments is a quadrilateral. A quadrilateral is also called a four-sided polygon, with four angles.

## Analysis

Move points A, B, C or D. Will these four points always determine a quadrilateral?

## ELEMENTS OF THE QUADRILATERALS

## CONVEX QUADRILATERAL AND CONCAVE QUADRILATERAL

## Analysis

Check the "Show / Hide internal angles" box. Move the vertices and observe what happens with the angles and the quadrilateral. Write another definition for convex and concave quadrilaterals.

## SUM OF THE INTERNAL ANGLES OF QUADRILATERALS

## Analysis

Check the option "Show / Hide diagonal" and demonstrate why the sum is always 360º.

## Analysis

Check the "Show / Hide diagonal" option and explain why the sum is always equal to 360º.