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1、Acute AngleAn angle whose measure is lessthan 90 degrees. AngleThe figure formed by two rays or line segments that have a common vertex. Angles are measured in degrees. The angle at point A on the triangle below is identified as angle BAC or &BAC. The sides of an angle are rays that have the vertex
2、as a starting point. Each of the three angles below is formed by the joining of two rays.Angle SumThe sum of all the measures of the interior angles of a polygon.Central AngleAn angle with a vertex at the center of a circle and whose sides are radii of the circle.DegreeA unit of measure of angles is
3、 also equal to 1/360 of a complete circle. The angle below measures about 1 degree; 360 of these would just fit around a point and fill in a complete circle; 90 of them make a right angle.DiagonalA line segment connecting two nonadjacent vertices of a polygon. All quadrilaterals have two diagonals,
4、as shown below. The two diagonals of a square are equal in length, and the two diagonals of a rectangle are equal in length. A pentagon has five diagonals. A hexagon has six diagonals.Equilateral TriangleA triangle with all three sides the same length.Exterior AngleAn angle at a vertex of a polygon
5、where the sides of the angle are one side of the polygon and the extension of the other side meeting at that vertex. Interior AngleThe angle inside a polygon formed by two adjacent sides of the polygon.Irregular PolygonA polygon which has at leasttwo sides with different lengths or two angles with d
6、ifferent measures.Isosceles TriangleA triangle with two sides the same length.Line of SymmetryA line such that if a shape isfolded over this line the two halves of the shape match exactly.Line of symmetryLine SegmentA line segment consists of two points of a line and all the points between these two
7、 points.MidpointThe point that divides a line segment into two segments of equal length.Obtuse AngleAn angle whose measure is greater than 90 and less than 180.Parallel LinesLines in a plane that never meet. Theopposite sides of a regular hexagon are parallel. ParallelogramA quadrilateral with oppos
8、ite sides parallel. Both pairs of opposite angles are also equal. In the definition of parallel lines, figure D, rectangle C, and square E are all parallelograms.Perpendicular LinesTwo lines that intersect toform right angles.PolygonA shape formed by line segments, called sides, so that each of the
9、segments meets exactly two other segments, and all of the points where the segments meet are endpoints of the segments.Special polygons Some special polygons use Greek prefixes that tell the number of sides or the number of angles in the polygon. triangle: A polygon with 3 sides and angles quadrilat
10、eral: A polygon with 4 sides and angles pentagon: A polygon with 5 sides and angles hexagon: A polygon with 6 sides and angles heptagon: A polygon with 7 sides and angles octagon: A polygon with 8 sides and angles nonagon (also called enneagon): A polygon with 9 sides and angles decagon: A polygon w
11、ith 10 sides and angles dodecagon: A polygon with 12 sides and angles QuadrilateralA polygon with four sides.RayA part of a line consisting of a point, called an endpoint, and all the points on the line on one side of the endpoint.RectangleA parallelogram with all right angles. Squares are a special
12、 type of rectangle.Reflection SymmetryA shape with reflectionsymmetry has two halves that are mirror images of each other.Regular PolygonA polygon that has all of its sides equal and all of its angles equal. The hexagon below is regular, but the pentagon is not regular, because its sides and its ang
13、les are not equal.RhombusA quadrilateral that has all sides the same length.Right AngleAn angle that measures 90. A rectangle has four right angles.Right TriangleA triangle with one right angle and two acute angles.Rotation SymmetryA shape has rotation symmetry if it can be rotated less than a full
14、turnabout its center point to a position where it looks exactly as it did before it was rotated.Scalene TriangleA triangle with no side lengths equal.SquareA rectangle with all sides equal. Squares have four right angles and four equal sides. Straight AngleAn angle that measures 180.TilingAlso calle
15、d a tessellation. The covering of a plane surface with geometric shapes without gaps or overlaps. These shapes are usually regular polygons or other common polygons. The tiling below is made of triangles. You could remove some of the line segments to create a tiling of parallelograms, or remove stil
16、l more to create a tiling of hexagons. In a tiling, a vertex is a point where the corners of the polygons fit together.TransversalA line that intersects two or more transversal lines. Lines s and t are transversals.TrapezoidA quadrilateral with at least one pair of opposite sides parallel. This definition means that parallelograms are trapezoidsVertexA corner of a polygon. For example, G, H, I, J, and K are all vertices in the pentagon below. All angles have vertices; for example, in the hexagon below, angle AFE has a vertex at F.
