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Geometry vocabulary (common core)

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plane

an endless flat surface

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parallel lines

lines that NEVER intersect

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intersecting lines

lines that pass through the same point

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perpendicular lines

lines that form right angles (perfect square corners; 90 degrees)

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line segment

a part of a line with two endpoints

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ray

a part of a line that has one endpoint and continues on forever in one direction

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right angle

measures 90 degrees; forms a perfect square corner

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acute angle

measures less than a right angle

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obtuse angle

measures greater than a right angle, but less than a straight angle

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straight angle

forms a straight line

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polygon

a closed plane figure made up of line segments

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side

a line segment of a polygon

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vertex

the point where two sides meet on a polygon

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quadrilateral

4-sided polygon

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equilateral triangle

has 3 equal sides

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isosceles triangle

has 2 equal sides

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scalene triangle

no equal sides

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right triangle

has 1 right angle

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acute triangle

has 3 acute angles

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obtuse triangle

has 1 obtuse angle

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parallelogram

has 2 pairs of parallel sides

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square

*has 4 right angles

*has 2 pairs of parallel sides

*all sides are equal

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rectangle

*has 4 right angles

*has 2 pairs of parallel sides

*opposite sides are equal in length

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rhombus

*has 2 pairs of parallel sides

*all sides are equal

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trapezoid

*has 1 pair of parallel sides

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symmetric

when a figure can be folded on a line to form two equal halves that fit on top of each other

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line of symmetry

the fold line