Unizor - Geometry2D - Trapezoid
Channel: Zor Shekhtman on UNIZOR Education
Date: 2011-10-22

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A quadrangle (a polygon with 4 vertices) with two opposite sides parallel to each other and two other sides not parallel to each other is called a trapezoid. 
Here are some fundamental properties of a trapezoid.
1.Bases of trapezoid are not congruent to each other (i.e. one must be shorter than the other).
2.In an isosceles trapezoid ABCD with bases AD and BC angles ∠ABC and ∠BCD, formed by base BC with two legs, are congruent to each other and, similarly, angles ∠BAD and ∠ADC, formed by base AD with two legs, are congruent to each other too.
3.In an isosceles trapezoid ABCD with bases AD and BC angles ∠ABC and ∠BAD, formed by leg AB with two bases, are supplemental and, similarly, angles ∠BCD and ∠CDA, formed by leg CD with two bases, are supplemental too.
4.Median of a trapezoid is parallel to its bases and equal to half sum of these two bases.
5.A line, connecting midpoints of two bases of an isosceles trapezoid, is its axis of symmetry.