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# Geometry Vocabulary

### Vocab for the project that Mrs. Hector says we have to know

TermDefinition
Acute Triangle A triangle for which all interior angles are acute, or less than 90 degrees.
Alternate exterior angles Exterior angles on alternate sides of the transversal (not on the same parallel line)
Alternate interior angles Interior angles on alternate sides of the transversal (not on the same parallel line)
Altitude of a triangle A straight line through a vertex and perpendicular to (ie forming a right angle with) a line containing the base (opposite side) of a triangle
Angle Bisector theorem Concerned with the relative lengths of the two segments that a triangle's side is divided into by a line that bisects the opposite angle. It equates their relative lengths to the relative lengths of the other two sides of the triangle
Converse of angle bisector theorem If a point is equidistant from the sides of an angle, then it is on the angle bisector
Angle formed by two rays, called the sides of the angle, sharing a common endpoint, called the vertex of the angle
Arc A closed segment of a differentiable curve in the 2D plane
Area Any particular extent of space or suface
Bisector A line that divides something into two equal parts
Center of a polygon In a rotation, the point that does not move. The rest of the plane rotates around this one fixed point
Centroid of a triangle the point where the three medians of the triangle intersect
Circumcenter of a triangle the point where the three perpendicular bisectors of a triangle meet
Circumference (circle) A complete circular arc; also the distance around the outside of the circle
Circumscribed A geometric figure that is drawn around another geometric figure so as to touch all its vertices
Combination A way of selecting several things out of a larger group, where (unlike permutations) order does not matter
Common parts Informal language that describes similarities, differences, parts (e.g. number of sides and vertices/"corners") and other attributes (e.g. having sides of equal lengths) of 2D and 3D shapes, in different sizes and orientaitons
Compass An instrument for drawing circles and arcs and measuring distances between points, consisting of two arms linked by a movable joint
Complement probability In probability theory, the complement of any event A is the event [not A], i.e. the event that A does not occur.
Complementary angle two angles that add up to 90 degrees (they form a right angle)
Composition the combining of distinct parts or element to form a whole
Compound event an event whose probability of occurrence depends upon the probability of occurrence of tow or more independent event
Compression to reduce a shape in size while retaining proportions
Conditional probability the probability that an event will occur, when another event is known to occur or to have occurred
Conditional probability formula the conditional probability of A given B is denoted by P(AB) and defined by the formula P(A|B)=P(A|B) P(B), provided P(B)>0
Congruence by AAS ASA SAS SSS Triangles are congruent if any pair of corresponding sides and their included angles are equal in both thingd
Congruent identical in form; coinciding exactly when superimposed
Construction The drawing of various shapes using only a compass or straightedge or ruler. No measurement of lengths or angles is allowed
Coordinates On the coordinate plane, the pair of numbers giving the location of a point (ordered pair). In 3D coordinates, the triple of numbers giving the location of a point (ordered triple). In n-dimensional space, a sequence of n numbers written is parentheses
Corresponding angles the angles in matching corners when two lines are crossed by another line (which is called transversal)
Dependent events when the outcome of one event effects the outcome of anther
Dilation A transformation that grows or shrinks a polygon by a given proportion about the center point
Equidistant Distant by equal amounts from two or more places
Endpoint Either of two points marking the end of a line segment
Events A set of outcomes of an experiment (subset of the sample space) to which a probability is assigned
Experimental probability The ratio of the number of times the event occurs to the total number of trials
Exterior angle the angle between any side of a polygon and an extended adjacent side
Frequency table Lists items and uses tally marks to record and show the number of times they occur
Fundamental counting principle When there are m ways to do one thing, and n ways to do another, then there are m x n ways of doing both
Glide reflection a transformation in which a graph or geometric figure is picked up and moved to another location without any change in size or orientation
Glide reflection symmetry the symmetry that a figure has if it can be made to fit exactly onto the original when it is translated a given distance at a given direction and then reflected over a line
horizontal line a constructive line, either drawn or imagined, which passes through the point of sight, and is the chief line in the projection upon which all verticals are fixed, and upon which all vanishing points are found
Hypotenuse the longest side of a right triangle; the opposite side of a right angle
Hypotenuse-leg theorem (HL) if the hypotenuse and leg of a right triangle are congruent to the hypotenuse and leg of another right triangle, then the triangles are congruent
image an optically formed duplicate, counterpart, or other representative reproduction of an object, especially an optical reproduction formed by a lens or mirror
Incenter or a triangle the point where the three angle bisectors of a triangle meet
included angle the angle made by two lines with a common vertex
Independent events when the outcome of an event does not influence the outcome of the second event
Inscribed in (the triangle) drawing one shape inside a triangle so that it just touches the sides of the triangle
interior angle any of the four angles formed between two straight lines intersected by a third straight line
intersection the probability that both A and B will occur
Isometry A transformation that is invariant with respect to distance. That is, the distance between any two points in the pre-image must be the same as the distance between the images of the two points
Isosceles triangle theorem the angles opposite the two equal sides of an isosceles triangle are =
Converse of Isosceles triangle theorem if two angles of an isosceles triangle are congruent, the sides opposite them are congruent
Leg either of the sides in a right triangle opposite an acute angle
line of symmetry The line of symmetry of a 2D figure is a line such that, for each perpendicular constructed, if the perpendicular intersects the figure at a distance d from the axis along the perpendicular, then there exist another the intersection of the figure and the
line of symmetry part II perpendicular, at the same distance d from the axis, in the opposite direction along the perpendicular
median of a triangle a line segment joining a vertex of a triangle to the midpoint of the opposing side
midpoint a point at or near the middle of, or equal distance from, both ends, as of one line, the midpoint of a boundary
Midpoint formula in the coordinate plane the point halfway between the endpoints of a line segment is called the midpoint. A midpoint divides a line segment into two equal segments
midsegment of a triangle the segment joining the midpoints of two sides of a triangle
Mutually exclusive events two events that cannot occur at the same time
n factorial the factorial of a natural number n is the product of the positive integers less than or = to n