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### Foundations for Geometry

acute angle An angle that measures less than 90 degrees.
acute triangle A triangle with all angles measuring less than 90 degrees.
angle A figure formed by two rays with a common vertex.
center of rotation The point about which a figure is rotated.
complementary angles Two angles whose measures add up to 90 degrees.
congruent Having the same size and shape.
correspondence The relationship between two or more objects that are matched.
equilateral triangle A triangle with three congruent sides.
image A figure resulting from a transformation.
isosceles triangle A triangle with at least two congruent sides.
line A straight path that extends without end in opposite directions.
line of symmetry the imaginary "mirror" in line symmetry.
line symmetry One half of a figure is a mirror image of the other half.
obtuse angle An angle whose measure is greater that 90 degrees but less than 180 degrees.
obtuse triangle A triangle containing one obtuse angle.
parallel lines Lines in a plane that do not intersect.
parallelogram A quadrilateral with two pairs of parallel sides.
perpendicular lines Lines that intersect to form right angles.
plane A flat surface that extends forever.
point An exact location in space.
polygon A close plane figure formed by three or more line segments that intersect only at their endpoints.
ray A part of a line that starts at one endpoint and extends forever.
rectangle A parallelogram with four right angles.
reflection A transformation of a figure that flips the figure across a line.
regular polygon A polygon with congruent sides and angles.
regular tessellation A regular polygon is repeated to fill a plane.
rhombus A parallelogram with all sides congruent.
right angle An angle that measures 90 degrees.
right triangle A triangle containing a right angle.
rise The vertical change when,the slope of a line is expressed as the ratio rise/run, or "rise over run".
rotation A transformation in which a figure is turned around a point.
rotational symmetry When a figure can be rotated less than 360 degrees around a central point and coincides with the original figure.
run The horizontal change when,the slope of a line is expressed as the ratio rise/run, or "rise over run".
scalene triangle A triangle with no congruent sides.
segment A part of a line between two endpoints.
slope A measure of the steepness of a line on a graph; the rise divided by the run.
square A rectangle with four congruent sides.
supplementary angles Two angles whose measures have a sum of 180 degrees.
tessellation A repeating pattern of plane figures that completely cover a plane with no gaps or overlaps.
transformation A change in the size or position of a figure.
translation A movement (slide) of a figure along a straight line.
transversal A line that intersects two or more lines.
trapezoid A quadrilateral with exactly one pair of parallel sides.
Triangle Sum Theorem The measures of the angles in a triangle add up to 180 degrees.
vertical angles A pair of opposite congruent angles formed by intersecting lines.
Created by: bookannelid