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In geometry, the **folium of Descartes** is an algebraic curve defined by the equation

**Geometry** is a branch of mathematics concerned with questions of shape, size, relative position of figures, and the properties of space. A mathematician who works in the field of geometry is called a geometer.

In mathematics, a **plane real algebraic curve** is the set of points on the Euclidean plane whose coordinates are zeros of some polynomial in two variables. More generally an **algebraic curve** is similar but may be embedded in a higher dimensional space or defined over some more general field.

- History
- Graphing the curve
- Relationship to the trisectrix of MacLaurin
- Notes
- References
- External links

- .

It forms a loop in the first quadrant with a double point at the origin and asymptote

In analytic geometry, an **asymptote** of a curve is a line such that the distance between the curve and the line approaches zero as one or both of the *x* or *y* coordinates tends to infinity. Some sources include the requirement that the curve may not cross the line infinitely often, but this is unusual for modern authors. In projective geometry and related contexts, an asymptote of a curve is a line which is tangent to the curve at a point at infinity.

- .

It is symmetrical about .

The name comes from the Latin word *folium* which means "leaf".

**Latin** is a classical language belonging to the Italic branch of the Indo-European languages. The Latin alphabet is derived from the Etruscan and Greek alphabets and ultimately from the Phoenician alphabet.

A **leaf** is an organ of a vascular plant and is the principal lateral appendage of the stem. The leaves and stem together form the shoot. Leaves are collectively referred to as **foliage,** as in "autumn foliage".

The curve was featured, along with a portrait of Descartes, on an Albanian stamp in 1966.

The curve was first proposed by Descartes in 1638. Its claim to fame lies in an incident in the development of calculus. Descartes challenged Fermat to find the tangent line to the curve at an arbitrary point since Fermat had recently discovered a method for finding tangent lines. Fermat solved the problem easily, something Descartes was unable to do.^{ [1] } Since the invention of calculus, the slope of the tangent line can be found easily using implicit differentiation.

**Calculus** is the mathematical study of continuous change, in the same way that geometry is the study of shape and algebra is the study of generalizations of arithmetic operations.

Since the equation is degree 3 in both x and y, and does not factor, it is difficult to solve for one of the variables.

However, the equation in polar coordinates is:

which can be plotted easily. By using this formula, the area of the interior of the loop is found to be .

Another technique is to write y = px and solve for x and y in terms of p. This yields the rational parametric equations:^{ [2] }

In mathematics, a **rational function** is any function which can be defined by a **rational fraction**, *i.e.* an algebraic fraction such that both the numerator and the denominator are polynomials. The coefficients of the polynomials need not be rational numbers; they may be taken in any field *K*. In this case, one speaks of a rational function and a rational fraction *over K*. The values of the variables may be taken in any field *L* containing *K*. Then the domain of the function is the set of the values of the variables for which the denominator is not zero and the codomain is *L*.

.

We can see that the parameter is related to the position on the curve as follows:

*p*< -1 corresponds to x>0, y<0: the right, lower, "wing".- -1 <
*p*< 0 corresponds to x<0, y>0: the left, upper "wing". *p*> 0 corresponds to x>0, y>0: the loop of the curve.

Another way of plotting the function can be derived from symmetry over y = x. The symmetry can be seen directly from its equation (x and y can be interchanged). By applying rotation of 45° CW for example, one can plot the function symmetric over rotated x axis.

This operation is equivalent to a substitution:

and yields

Plotting in the cartesian system of (u,v) gives the folium rotated by 45° and therefore symmetric by u axis.

Since the folium is symmetrical about , it passes through the point .

The folium of Descartes is related to the trisectrix of Maclaurin by affine transformation. To see this, start with the equation

In geometry, the **trisectrix of Maclaurin** is a cubic plane curve notable for its trisectrix property, meaning it can be used to trisect an angle. It can be defined as locus of the point of intersection of two lines, each rotating at a uniform rate about separate points, so that the ratio of the rates of rotation is 1:3 and the lines initially coincide with the line between the two points. A generalization of this construction is called a sectrix of Maclaurin. The curve is named after Colin Maclaurin who investigated the curve in 1742.

In geometry, an **affine transformation**, **affine map** or an **affinity** is a function between affine spaces which preserves points, straight lines and planes. Also, sets of parallel lines remain parallel after an affine transformation. An affine transformation does not necessarily preserve angles between lines or distances between points, though it does preserve ratios of distances between points lying on a straight line.

- ,

and change variables to find the equation in a coordinate system rotated 45 degrees. This amounts to setting . In the plane the equation is

- .

If we stretch the curve in the direction by a factor of this becomes

which is the equation of the trisectrix of Maclaurin.

- ↑ Simmons, p. 101
- ↑ "DiffGeom3: Parametrized curves and algebraic curves". N J Wildberger, University of New South Wales . Retrieved 5 September 2013.

In classical mathematics, **analytic geometry**, also known as **coordinate geometry** or **Cartesian geometry**, is the study of geometry using a coordinate system. This contrasts with synthetic geometry.

In mathematics, the **polar coordinate system** is a two-dimensional coordinate system in which each point on a plane is determined by a distance from a reference point and an angle from a reference direction.

**Snell's law** is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water, glass, or air.

A **tautochrone** or **isochrone curve** is the curve for which the time taken by an object sliding without friction in uniform gravity to its lowest point is independent of its starting point. The curve is a cycloid, and the time is equal to π times the square root of the radius over the acceleration of gravity. The tautochrone curve is the same as the brachistochrone curve for any given starting point.

In the mathematical field of differential geometry, a **metric tensor** is a type of function which takes as input a pair of tangent vectors v and w at a point of a surface and produces a real number scalar *g*(*v*, *w*) in a way that generalizes many of the familiar properties of the dot product of vectors in Euclidean space. In the same way as a dot product, metric tensors are used to define the length of and angle between tangent vectors. Through integration, the metric tensor allows one to define and compute the length of curves on the manifold.

In geometry, a **limaçon** or **limacon**, also known as a **limaçon of Pascal**, is defined as a roulette formed by the path of a point fixed to a circle when that circle rolls around the outside of a circle of equal radius. It can also be defined as the roulette formed when a circle rolls around a circle with half its radius so that the smaller circle is inside the larger circle. Thus, they belong to the family of curves called centered trochoids; more specifically, they are epitrochoids. The **cardioid** is the special case in which the point generating the roulette lies on the rolling circle; the resulting curve has a cusp.

In geometry, the **cissoid of Diocles** is a cubic plane curve notable for the property that it can be used to construct two mean proportionals to a given ratio. In particular, it can be used to double a cube. It can be defined as the cissoid of a circle and a line tangent to it with respect to the point on the circle opposite to the point of tangency. In fact, the family of cissoids is named for this example and some authors refer to it simply as the cissoid. It has a single cusp at the pole, and is symmetric about the diameter of the circle which is the line of tangency of the cusp. The line is an asymptote. It is a member of the conchoid of de Sluze family of curves and in form it resembles a tractrix.

The **pedal curve** results from the orthogonal projection of a fixed point on the tangent lines of a given curve. More precisely, for a plane curve *C* and a given fixed *pedal point**P*, the **pedal curve** of *C* is the locus of points *X* so that the line *PX* is perpendicular to a tangent *T* to the curve passing through the point *X*. Conversely, at any point *R* on the curve *C*, let *T* be the tangent line at that point *R*; then there is a unique point *X* on the tangent *T* which forms with the pedal point *P* a line perpendicular to the tangent *T* – the pedal curve is the set of such points *X*, called the *foot* of the perpendicular to the tangent *T* from the fixed point *P*, as the variable point *R* ranges over the curve *C*.

The **conchoid(s) of de Sluze** is a family of plane curves studied in 1662 by René François Walter, baron de Sluze.

In geometry, a **cissoid** is a curve generated from two given curves *C*_{1}, *C*_{2} and a point *O*. Let *L* be a variable line passing through *O* and intersecting *C*_{1} at *P*_{1} and *C*_{2} at *P*_{2}. Let P be the point on L so that *OP* = *P*_{1}*P*_{2}. Then the locus of such points *P* is defined to be the cissoid of the curves *C*_{1}, *C*_{2} relative to *O*.

**Arc length** is the distance between two points along a section of a curve.

In geometry, a **strophoid** is a curve generated from a given curve *C* and points *A* and *O* as follows: Let *L* be a variable line passing through *O* and intersecting *C* at *K*. Now let *P*_{1} and *P*_{2} be the two points on *L* whose distance from *K* is the same as the distance from *A* to *K*. The locus of such points *P*_{1} and *P*_{2} is then the strophoid of C with respect to the pole *O* and fixed point *A*. Note that *AP*_{1} and *AP*_{2} are at right angles in this construction.

In mathematics, **Watt's curve** is a tricircular plane algebraic curve of degree six. It is generated by two circles of radius *b* with centers distance 2*a* apart (taken to be at. A line segment of length 2*c* attaches to a point on each of the circles, and the midpoint of the line segment traces out the Watt curve as the circles rotate. It arose in connection with James Watt's pioneering work on the steam engine.

A **parametric surface** is a surface in the Euclidean space which is defined by a parametric equation with two parameters Parametric representation is a very general way to specify a surface, as well as implicit representation. Surfaces that occur in two of the main theorems of vector calculus, Stokes' theorem and the divergence theorem, are frequently given in a parametric form. The curvature and arc length of curves on the surface, surface area, differential geometric invariants such as the first and second fundamental forms, Gaussian, mean, and principal curvatures can all be computed from a given parametrization.

In mathematics, a **rotation of axes** in two dimensions is a mapping from an *xy*-Cartesian coordinate system to an *x'y'*-Cartesian coordinate system in which the origin is kept fixed and the *x'* and *y'* axes are obtained by rotating the *x* and *y* axes counterclockwise through an angle . A point *P* has coordinates with respect to the original system and coordinates with respect to the new system. In the new coordinate system, the point *P* will appear to have been rotated in the opposite direction, that is, clockwise through the angle . A rotation of axes in more than two dimensions is defined similarly. A rotation of axes is a linear map and a rigid transformation.

In inversive geometry, an **inverse curve** of a given curve C is the result of applying an inverse operation to C. Specifically, with respect to a fixed circle with center O and radius k the inverse of a point Q is the point P for which P lies on the ray OQ and *OP*·*OQ* = *k*^{2}. The inverse of the curve C is then the locus of P as Q runs over C. The point O in this construction is called the **center of inversion**, the circle the **circle of inversion**, and k the **radius of inversion**.

In geometry, a **limaçon trisectrix** is a member of the limaçon family of curves which has the trisectrix, or angle trisection, property. It can be defined as locus of the point of intersection of two lines, each rotating at a uniform rate about separate points, so that the ratio of the rates of rotation is 2:3 and the lines initially coincide with the line between the two points. Thus, it is an example of a sectrix of Maclaurin.

In geometry, a **sectrix of Maclaurin** is defined as the curve swept out by the point of intersection of two lines which are each revolving at constant rates about different points called **poles**. Equivalently, a sectrix of Maclaurin can be defined as a curve whose equation in biangular coordinates is linear. The name is derived from the trisectrix of Maclaurin, which is a prominent member of the family, and their sectrix property, which means they can be used to divide an angle into a given number of equal parts. There are special cases are also known as **arachnida** or **araneidans** because of their spider-like shape, and **Plateau curves** after Joseph Plateau who studied them.

For a plane curve *C* and a given fixed point *O*, the **pedal equation** of the curve is a relation between *r* and *p* where *r* is the distance from *O* to a point on *C* and *p* is the perpendicular distance from *O* to the tangent line to *C* at the point. The point *O* is called the *pedal point* and the values *r* and *p* are sometimes called the *pedal coordinates* of a point relative to the curve and the pedal point. It is also useful to measure the distance of *O* to the normal even though it is not an independent quantity and it relates to as .

- J. Dennis Lawrence:
*A catalog of special plane curves*, 1972, Dover Publications. ISBN 0-486-60288-5, pp. 106–108 - George F. Simmons:
*Calculus Gems: Brief Lives and Memorable Mathematics*, New York 1992, McGraw-Hill, xiv,355. ISBN 0-07-057566-5; new edition 2007, The Mathematical Association of America (MAA)

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