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Point

A specific place, marked by a dot and usually named with a letter.

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Line

A straight path that goes on forever in both directions. Named using two points on the line. A line has arrows on both ends that show it keeps going.

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Line Segment

A part of a line that has two endpoints. A line segment is named using the two endpoints.

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Ray

A ray has an endpoint on one side and keeps going forever on the other. You always name a Ray using the endpoint first.

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plane

A flat surface that keeps spreading in all directions forever. Must be named by three points on the plane.

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Intersecting lines

Two lines that cross

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Parallel lines

Two lines, that lie next to each other without ever getting close together or further apart.

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Perpendicular lines

Two lines that make right angles when they cross.

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degrees

The units used to measure an angle.

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Right angle

An angle that measures 90 degrees.

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Acute angle

An angle that measures less than 90 degrees. (Babies are small, babies are cute. An acute angle is small like a baby)

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Obtuse angle

An angle that is more than 90 degrees. (Obtuse can also mean slow. An obtuse angle is a big, slow angle.)

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Straight angle

An angle that measures 180 degrees. This is a flat angle. It looks like a line.

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vertex

The vertex is the point where two rays or line segments meet to form an angle.

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Equilateral Triangle

A triangle that has all three sides the same length.

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Isosceles Triangle

A triangle with two sides that have the same length.

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Scalene Triangle

A triangle with no equal length sides.

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Right Triangle

A triangle that has a 90 degree, or right, angle.

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Acute Triangle

A triangle that has three interior angles that are less than 90 degrees.

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Obtuse Triangle

A triangle that has one interior angle that is greater than 90 degrees.

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Interior Angle

An angle found on the inside of a shape or object.

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Exterior Angle

An angle found on the outside of a shape or object.

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Congruent

Two shapes are congruent when all of the sides and angles of the two shapes are exactly the same. The object can be rotated and face a different direction, but the measurements must be the same.

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Similar

The angles and the overall appearance of the shapes are the same, but one object is larger than the other.

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Quadrilateral

Any shape that is a polygon and has four sides.

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Polygon

Any object made of at least three straight lines. There can be no gaps, no curves, and no lines that cross each other.

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Rectangle

Opposite sides are parallel and congruent. Shape has four right angles.

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Square

All four sides are congruent and parallel. There are four right angles.

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parallelogram

Four sided shape. Opposite sides are congruent and parallel.

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rhombus

A four sided shape with all sides congruent and opposite sides parallel.

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trapezoid

A four sided shape with only one set of parallel lines.

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Regular Polygon

A shape with congruent sides and identical angles.

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Irregular Polygon

A shape made up of sides that are different links and unlike angles.

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Pentagon

Five sided shape.

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Heptagon

Seven sided shape.

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Octagon

Eight sided shape.

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Nonagon

Nine sided shape.

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Undecagon

Eleven sided shape.

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Dodecagon

Twelve sided shape.

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Diagonal

A line segment that joins two vertices of a shape by going through the middle of the shape.

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Center (of a circle)

A point in the middle of a circle that is the same distance from outside of the circle all the way around.

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radius

A line segment that connects the center of the circle to a point on the circle.

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diameter

A line segment that goes from one side of the circle to the other and passes through the center of the circle.

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Chord

A line segment that goes from one side of the circle to the other side. A chord does not have to go through the center.

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Central Angle

An angle, whose vertex is at the center of a circle.

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Line Symmetry

If an object can have a line drawn down the center and both sides are congruent, then the object has line symmetry.

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Rotational Symmetry

If you can rotate an object less than 360 degrees and the object looks like it did before you turned it, then the object has rotational symmetry.

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Circumference

The distance around the outside of a circle

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Complementary Angle

two angles that add up to 90 degrees

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Supplementary Angle

two angles whose measures have a sum of 180 degrees

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