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This essential trains for: Math Kangaroo 5-6, Math Kangaroo 7-8, SAT-I, SAT-II, GMAT, AMC-8.

A quadrilateral is a polygon with four sides. It may be convex, concave, and may or may not have symmetry features.

A quadrilateral with all congruent sides is a rhombus.

A rhombus with all congruent angles is a square.

A quadrilateral with two adjacent congruent sides and adjacent angles that add up to 180° is a kite.

A quadrilateral with two opposite congruent sides and all congruent angles is a rectangle.

A quadrilateral with two parallel sides and two non-parallel sides is a trapezoid.

A quadrilateral with each pair of opposite sides parallel is a parallelogram.

A quadrilateral that can be inscribed in a circle is a cyclic quadrilateral.

Theorem: The diagonals of a parallelogram bisect each other (intersect at each other's midpoints).