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

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Logarithmic Spiral
** -
A polar equation of the form *r* = *ab*^{θ}.

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Orientation
** -
The direction of a plane curve as the parameter increases.

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

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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*.

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

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

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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.

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

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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.

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

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Spiral of Archimedes
** -
A polar equation of the form *r* = *aθ* + *b*.