Geometry Glossary - R

Radian:

a unit of angular measure equal to the measure of a central angle intercepting a one-unit long arc on the unit circle

Radical:

 a square root

Radicand:

 the number under the radical sign

Radius:

a line segment that joins the center of the circle to a point on the circle

also, a line segment that joins the center of a regular polygon to a vertex of the polygon

Ray:

"half of a line;" the union of a segment and all the points (on the line containing the segment) beyond one of the endpoints of the segment

Rectangle:

 a quadrilateral with four right angles (a rectangle is a special kind of parallelogram)

Rectangular Solid: (box)

the union of a prism whose bases and lateral faces are rectangles and the space it encloses

Reference angle:

an acute in the first quadrant, with the vertex at the origin, which is used to determine the trigonometric functions of angles in the other quadrants

Reflection:

the image created by flipping a point (or set of points) over a line to a new position on the opposite side of the line, as if looking in a mirror

see also: transformations

Regular (when used in the name of a surface):

adjective indicating that the base(s) is a regular polygon; a regular prism or pyramid is also right

Regular polygon:

a convex polygon that is both equilateral and equiangular

Regular Pyramid:

a pyramid that has a regular polygon for a base, and whose altitude has endpoints at the pyramid's vertex and at the center of the base

Remote interior angles:

the angles in a triangle that are not adjacent to a given exterior angle

Rhombus:

 a parallelogram with four equal sides; a square is a special type of rhombus

Right (when used in the name of a surface):

adjective indicating either that the parallel bases are aligned (for prisms and cylinders), or that the tip is centered over the base (for pyramids and cones); a right prism will have rectangular lateral faces; a right pyramid will have lateral faces, which are isosceles triangles

Right angle:

an angle whose measure is 90°

Right cone:

a cone in which the endpoints of the altitude are the vertex and the center of the base

Right cylinder:

a cylinder whose axis is also an altitude

Right prism:

a prism in which the lateral surfaces are perpendicular to the bases

Right triangle:

a triangle in which one of the angles measures 90°

Rotation:

a figure is rotated if every point moves through a given number of degrees around a given point; (a rotation is an example of a rigid transformation) a rotation is also defined as composition of two reflections over intersecting lines of reflection