Quadrilateral-Basic Notions

Quadrilateral - Definition
[justify]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. [/justify]
Analysis
Move points A, B, C or D. Will these four points always determine a quadrilateral?[br]
ELEMENTS OF THE QUADRILATERALS
CONVEX QUADRILATERAL AND CONCAVE QUADRILATERAL
Analysis
[justify]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.[/justify]
SUM OF THE INTERNAL ANGLES OF QUADRILATERALS
Analysis
Check the option "Show / Hide diagonal" and demonstrate why the sum is always 360º.[br]
Analysis
Check the "Show / Hide diagonal" option and explain why the sum is always equal to 360º.
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Information: Quadrilateral-Basic Notions