Quadrilaterals |
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A Quadrilateral is a closed figure with four sides, and four vertices. |

Figure 1. |
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Above is quadrilateral ABCD.

All four angles in a quadrilateral sum to 360 degrees.

So, Angle A + Angle B + Angle C + Angle D = 360 degrees.

Specific names can be given to certain types of quadrilaterals. You're probably already familiar with some of them:

Parallelogram |
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In a parallelogram: -all opposite sides are parallel -all opposite sides are congruent |

Trapezoid |
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In a trapezoid: -one pair of opposite sides is parallel |

Rectangle |
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In a rectangle: -all opposite sides are parallel -all opposite sides are congruent -contains four right angles |

Rhombus |
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In a rhombus: -all opposite sides are parallel -all sides are congruent |