• ### Limacon

A polar equation of the form r = a + b sin(θ) or r = a + b cos(θ), where a, b≠ 0.

• ### Logarithmic Spiral

A polar equation of the form r = abθ.

• ### Orientation

The direction of a plane curve as the parameter increases.

• ### Parameter

A third variable (often time) which determines the values of x and y in parametric equations.

• ### Parametric Equations

Two equations of the form x = f (t) and y = g(t), which specify the location of a point according to the variable t.

• ### Plane Curve

The set of all points (f (t), g(t)), where x = f (t) and y = g(t) are parametric equations.

• ### Polar Axis

The ray whose endpoint is the pole and which is the initial side of any angle measure in the polar plane.

• ### Polar Coordinate System

The system in which a point in the plane is specified according to an ordered pair (r, θ) in which r is a length and θ is an angle. The length r refers to the distance from the point to a fixed origin, called the pole. The angle θ is the angle whose initial side is a fixed ray (the polar axis) and whose terminal side contains the point. Under these circumstances, the point (r, θ) is expressed in polar coordinates.

• ### Pole

The fixed point in the polar coordinate system from which every point is r units away.

• ### Rectangular Coordinate System

The coordinate system in which every point is specified by exactly one ordered pair (x, y). Here x is the distance between the point and a fixed line (the y-axis) and y is the distance between the point and a line fixed perpendicular to the other line (this line is the x-axis). The perpendicular lines are the axes, and the point (x, y) is expressed in rectangular coordinates.

• ### Rose Curve

A polar equation of the form r = a sin() or r = a cos(), where n is an integer.

• ### Spiral of Archimedes

A polar equation of the form r = + b.