In geometry, the **tangent line** (or simply **tangent**) to a plane curve at a given point is the straight line that "just touches" the curve at that point. Leibniz defined it as the line through a pair of infinitely close points on the curve.^{ [1] } More precisely, a straight line is said to be a tangent of a curve *y* = *f*(*x*) at a point *x* = *c* if the line passes through the point (*c*, *f*(*c*)) on the curve and has slope *f*'(*c*), where *f*' is the derivative of *f*. A similar definition applies to space curves and curves in *n*-dimensional Euclidean space.

- History
- Tangent line to a curve
- Analytical approach
- Equations
- Normal line to a curve
- Angle between curves
- Multiple tangents at a point
- Tangent circles
- Surfaces and higher-dimensional manifolds
- See also
- References
- Sources
- External links

As it passes through the point where the tangent line and the curve meet, called the **point of tangency**, the tangent line is "going in the same direction" as the curve, and is thus the best straight-line approximation to the curve at that point.

Similarly, the **tangent plane** to a surface at a given point is the plane that "just touches" the surface at that point. The concept of a tangent is one of the most fundamental notions in differential geometry and has been extensively generalized; see Tangent space.

The word "tangent" comes from the Latin * tangere *, "to touch".

Euclid makes several references to the tangent (ἐφαπτομένη*ephaptoménē*) to a circle in book III of the * Elements * (c. 300 BC).^{ [2] } In Apollonius work *Conics* (c. 225 BC) he defines a tangent as being *a line such that no other straight line could* fall between it and the curve*. ^{ [3] }*

Archimedes (c. 287 – c. 212 BC) found the tangent to an Archimedean spiral by considering the path of a point moving along the curve.^{ [3] }

In the 1630s Fermat developed the technique of adequality to calculate tangents and other problems in analysis and used this to calculate tangents to the parabola. The technique of adequality is similar to taking the difference between and and dividing by a power of . Independently Descartes used his method of normals based on the observation that the radius of a circle is always normal to the circle itself.^{ [4] }

These methods led to the development of differential calculus in the 17th century. Many people contributed. Roberval discovered a general method of drawing tangents, by considering a curve as described by a moving point whose motion is the resultant of several simpler motions.^{ [5] } René-François de Sluse and Johannes Hudde found algebraic algorithms for finding tangents.^{ [6] } Further developments included those of John Wallis and Isaac Barrow, leading to the theory of Isaac Newton and Gottfried Leibniz.

An 1828 definition of a tangent was "a right line which touches a curve, but which when produced, does not cut it".^{ [7] } This old definition prevents inflection points from having any tangent. It has been dismissed and the modern definitions are equivalent to those of Leibniz who defined the tangent line as the line through a pair of infinitely close points on the curve.

The intuitive notion that a tangent line "touches" a curve can be made more explicit by considering the sequence of straight lines (secant lines) passing through two points, *A* and *B*, those that lie on the function curve. The tangent at *A* is the limit when point *B* approximates or tends to *A*. The existence and uniqueness of the tangent line depends on a certain type of mathematical smoothness, known as "differentiability." For example, if two circular arcs meet at a sharp point (a vertex) then there is no uniquely defined tangent at the vertex because the limit of the progression of secant lines depends on the direction in which "point *B*" approaches the vertex.

At most points, the tangent touches the curve without crossing it (though it may, when continued, cross the curve at other places away from the point of tangent). A point where the tangent (at this point) crosses the curve is called an * inflection point *. Circles, parabolas, hyperbolas and ellipses do not have any inflection point, but more complicated curves do have, like the graph of a cubic function, which has exactly one inflection point, or a sinusoid, which has two inflection points per each period of the sine.

Conversely, it may happen that the curve lies entirely on one side of a straight line passing through a point on it, and yet this straight line is not a tangent line. This is the case, for example, for a line passing through the vertex of a triangle and not intersecting it otherwise—where the tangent line does not exist for the reasons explained above. In convex geometry, such lines are called supporting lines.

The geometrical idea of the tangent line as the limit of secant lines serves as the motivation for analytical methods that are used to find tangent lines explicitly. The question of finding the tangent line to a graph, or the **tangent line problem,** was one of the central questions leading to the development of calculus in the 17th century. In the second book of his * Geometry *, René Descartes ^{ [8] } said of the problem of constructing the tangent to a curve, "And I dare say that this is not only the most useful and most general problem in geometry that I know, but even that I have ever desired to know".^{ [9] }

Suppose that a curve is given as the graph of a function, *y* = *f*(*x*). To find the tangent line at the point *p* = (*a*, *f*(*a*)), consider another nearby point *q* = (*a* + *h*, *f*(*a* + *h*)) on the curve. The slope of the secant line passing through *p* and *q* is equal to the difference quotient

As the point *q* approaches *p*, which corresponds to making *h* smaller and smaller, the difference quotient should approach a certain limiting value *k*, which is the slope of the tangent line at the point *p*. If *k* is known, the equation of the tangent line can be found in the point-slope form:

To make the preceding reasoning rigorous, one has to explain what is meant by the difference quotient approaching a certain limiting value *k*. The precise mathematical formulation was given by Cauchy in the 19th century and is based on the notion of limit. Suppose that the graph does not have a break or a sharp edge at *p* and it is neither plumb nor too wiggly near *p*. Then there is a unique value of *k* such that, as *h* approaches 0, the difference quotient gets closer and closer to *k*, and the distance between them becomes negligible compared with the size of *h*, if *h* is small enough. This leads to the definition of the slope of the tangent line to the graph as the limit of the difference quotients for the function *f*. This limit is the derivative of the function *f* at *x* = *a*, denoted *f* ′(*a*). Using derivatives, the equation of the tangent line can be stated as follows:

Calculus provides rules for computing the derivatives of functions that are given by formulas, such as the power function, trigonometric functions, exponential function, logarithm, and their various combinations. Thus, equations of the tangents to graphs of all these functions, as well as many others, can be found by the methods of calculus.

Calculus also demonstrates that there are functions and points on their graphs for which the limit determining the slope of the tangent line does not exist. For these points the function *f* is *non-differentiable*. There are two possible reasons for the method of finding the tangents based on the limits and derivatives to fail: either the geometric tangent exists, but it is a vertical line, which cannot be given in the point-slope form since it does not have a slope, or the graph exhibits one of three behaviors that precludes a geometric tangent.

The graph *y* = *x*^{1/3} illustrates the first possibility: here the difference quotient at *a* = 0 is equal to *h*^{1/3}/*h* = *h*^{−2/3}, which becomes very large as *h* approaches 0. This curve has a tangent line at the origin that is vertical.

The graph *y* = *x*^{2/3} illustrates another possibility: this graph has a * cusp * at the origin. This means that, when *h* approaches 0, the difference quotient at *a* = 0 approaches plus or minus infinity depending on the sign of *x*. Thus both branches of the curve are near to the half vertical line for which *y*=0, but none is near to the negative part of this line. Basically, there is no tangent at the origin in this case, but in some context one may consider this line as a tangent, and even, in algebraic geometry, as a *double tangent*.

The graph *y* = |*x*| of the absolute value function consists of two straight lines with different slopes joined at the origin. As a point *q* approaches the origin from the right, the secant line always has slope 1. As a point *q* approaches the origin from the left, the secant line always has slope −1. Therefore, there is no unique tangent to the graph at the origin. Having two different (but finite) slopes is called a *corner*.

Finally, since differentiability implies continuity, the contrapositive states *discontinuity* implies non-differentiability. Any such jump or point discontinuity will have no tangent line. This includes cases where one slope approaches positive infinity while the other approaches negative infinity, leading to an infinite jump discontinuity

When the curve is given by *y* = *f*(*x*) then the slope of the tangent is so by the point–slope formula the equation of the tangent line at (*X*, *Y*) is

where (*x*, *y*) are the coordinates of any point on the tangent line, and where the derivative is evaluated at .^{ [10] }

When the curve is given by *y* = *f*(*x*), the tangent line's equation can also be found^{ [11] } by using polynomial division to divide by ; if the remainder is denoted by , then the equation of the tangent line is given by

When the equation of the curve is given in the form *f*(*x*, *y*) = 0 then the value of the slope can be found by implicit differentiation, giving

The equation of the tangent line at a point (*X*,*Y*) such that *f*(*X*,*Y*) = 0 is then^{ [10] }

This equation remains true if but (in this case the slope of the tangent is infinite). If the tangent line is not defined and the point (*X*,*Y*) is said to be singular.

For algebraic curves, computations may be simplified somewhat by converting to homogeneous coordinates. Specifically, let the homogeneous equation of the curve be *g*(*x*, *y*, *z*) = 0 where *g* is a homogeneous function of degree *n*. Then, if (*X*, *Y*, *Z*) lies on the curve, Euler's theorem implies

It follows that the homogeneous equation of the tangent line is

The equation of the tangent line in Cartesian coordinates can be found by setting *z*=1 in this equation.^{ [12] }

To apply this to algebraic curves, write *f*(*x*, *y*) as

where each *u*_{r} is the sum of all terms of degree *r*. The homogeneous equation of the curve is then

Applying the equation above and setting *z*=1 produces

as the equation of the tangent line.^{ [13] } The equation in this form is often simpler to use in practice since no further simplification is needed after it is applied.^{ [12] }

If the curve is given parametrically by

then the slope of the tangent is

giving the equation for the tangent line at as^{ [14] }

If the tangent line is not defined. However, it may occur that the tangent line exists and may be computed from an implicit equation of the curve.

The line perpendicular to the tangent line to a curve at the point of tangency is called the *normal line* to the curve at that point. The slopes of perpendicular lines have product −1, so if the equation of the curve is *y* = *f*(*x*) then slope of the normal line is

and it follows that the equation of the normal line at (X, Y) is

Similarly, if the equation of the curve has the form *f*(*x*, *y*) = 0 then the equation of the normal line is given by^{ [15] }

If the curve is given parametrically by

then the equation of the normal line is^{ [14] }

The angle between two curves at a point where they intersect is defined as the angle between their tangent lines at that point. More specifically, two curves are said to be tangent at a point if they have the same tangent at a point, and orthogonal if their tangent lines are orthogonal.^{ [16] }

The formulas above fail when the point is a singular point. In this case there may be two or more branches of the curve that pass through the point, each branch having its own tangent line. When the point is the origin, the equations of these lines can be found for algebraic curves by factoring the equation formed by eliminating all but the lowest degree terms from the original equation. Since any point can be made the origin by a change of variables (or by translating the curve) this gives a method for finding the tangent lines at any singular point.

For example, the equation of the limaçon trisectrix shown to the right is

Expanding this and eliminating all but terms of degree 2 gives

which, when factored, becomes

So these are the equations of the two tangent lines through the origin.^{ [17] }

When the curve is not self-crossing, the tangent at a reference point may still not be uniquely defined because the curve is not differentiable at that point although it is differentiable elsewhere. In this case the left and right derivatives are defined as the limits of the derivative as the point at which it is evaluated approaches the reference point from respectively the left (lower values) or the right (higher values). For example, the curve *y* = |*x* | is not differentiable at *x* = 0: its left and right derivatives have respective slopes −1 and 1; the tangents at that point with those slopes are called the left and right tangents.^{ [18] }

Sometimes the slopes of the left and right tangent lines are equal, so the tangent lines coincide. This is true, for example, for the curve *y* = *x*^{2/3}, for which both the left and right derivatives at *x* = 0 are infinite; both the left and right tangent lines have equation *x* = 0.

Two circles of non-equal radius, both in the same plane, are said to be tangent to each other if they meet at only one point. Equivalently, two circles, with radii of *r _{i}* and centers at (

- Two circles are
**externally tangent**if the distance between their centres is equal to the sum of their radii.

- Two circles are
**internally tangent**if the distance between their centres is equal to the difference between their radii.^{ [19] }

The *tangent plane* to a surface at a given point *p* is defined in an analogous way to the tangent line in the case of curves. It is the best approximation of the surface by a plane at *p*, and can be obtained as the limiting position of the planes passing through 3 distinct points on the surface close to *p* as these points converge to *p*. More generally, there is a *k*-dimensional tangent space at each point of a *k*-dimensional manifold in the *n*-dimensional Euclidean space.

- Newton's method
- Normal (geometry)
- Osculating circle
- Osculating curve
- Perpendicular
- Subtangent
- Supporting line
- Tangent cone
- Tangential angle
- Tangential component
- Tangent lines to circles
- Multiplicity (mathematics)#Behavior of a polynomial function near a multiple root
- Algebraic curve#Tangent at a point

In calculus, the **chain rule** is a formula to compute the derivative of a composite function. That is, if f and g are differentiable functions, then the chain rule expresses the derivative of their composite *f*∘*g* — the function which maps *x* to — in terms of the derivatives of *f* and *g* and the product of functions as follows:

In vector calculus, the **curl** is a vector operator that describes the infinitesimal rotation of a vector field in three-dimensional Euclidean space. At every point in the field, the curl of that point is represented by a vector. The attributes of this vector characterize the rotation at that point.

The **derivative** of a function of a real variable which measures the sensitivity to change of the function value with respect to a change in its argument. Derivatives are a fundamental tool of calculus. For example, the derivative of the position of a moving object with respect to time is the object's velocity: this measures how quickly the position of the object changes when time advances.

In vector calculus, the **gradient** of a scalar-valued differentiable function *f* of several variables is the vector field whose value at a point is the vector whose components are the partial derivatives of at . That is, for , its gradient is defined at the point in *n-*dimensional space as the vector:

In mathematics, the **slope** or **gradient** of a line is a number that describes both the *direction* and the *steepness* of the line. Slope is often denoted by the letter *m*; there is no clear answer to the question why the letter *m* is used for slope, but its earliest use in English appears in O'Brien (1844) who wrote the equation of a straight line as "*y* = *mx* + *b*" and it can also be found in Todhunter (1888) who wrote it as "*y* = *mx* + *c*".

In mathematics, **differential calculus** is a subfield of calculus that studies the rates at which quantities change. It is one of the two traditional divisions of calculus, the other being integral calculus—the study of the area beneath a curve.

In mathematics, **curvature** is any of several strongly related concepts in geometry. Intuitively, the curvature is the amount by which a curve deviates from being a straight line, or a surface deviates from being a plane.

The **calculus of variations** is a field of mathematical analysis that uses variations, which are small changes in functions and functionals, to find maxima and minima of functionals: mappings from a set of functions to the real numbers. Functionals are often expressed as definite integrals involving functions and their derivatives. Functions that maximize or minimize functionals may be found using the Euler–Lagrange equation of the calculus of variations.

In mathematics, an **implicit equation** is a relation of the form , where is a function of several variables. For example, the implicit equation of the unit circle is .

In mathematics and physics, the **Legendre transformation**, named after Adrien-Marie Legendre, is an involutive transformation on the real-valued convex functions of one real variable. In physical problems, it is used to convert functions of one quantity into functions of the conjugate quantity. In this way, it is commonly used in classical mechanics to derive the Hamiltonian formalism out of the Lagrangian formalism and in thermodynamics to derive the thermodynamic potentials, as well as in the solution of differential equations of several variables.

A **cardioid** is a plane curve traced by a point on the perimeter of a circle that is rolling around a fixed circle of the same radius. It can also be defined as an epicycloid having a single cusp. It is also a type of sinusoidal spiral, and an inverse curve of the parabola with the focus as the center of inversion.

In mathematics, the **method of characteristics** is a technique for solving partial differential equations. Typically, it applies to first-order equations, although more generally the method of characteristics is valid for any hyperbolic partial differential equation. The method is to reduce a partial differential equation to a family of ordinary differential equations along which the solution can be integrated from some initial data given on a suitable hypersurface.

In geometry, an **envelope** of a planar family of curves is a curve that is tangent to each member of the family at some point, and these points of tangency together form the whole envelope. Classically, a point on the envelope can be thought of as the intersection of two "infinitesimally adjacent" curves, meaning the limit of intersections of nearby curves. This idea can be generalized to an envelope of surfaces in space, and so on to higher dimensions.

The solutions of a first-order differential equation of a scalar function y(x) can be drawn in a 2-dimensional space with the x in horizontal and y in vertical direction. Possible solutions are functions y(x) drawn as solid curves. Sometimes it is too cumbersome solving the differential equation analytically. Then one can still draw the tangents of the function curves e.g. on a regular grid. The tangents are touching the functions at the grid points. However, the direction field is rather agnostic about chaotic aspects of the differential equation.

In calculus, the **second derivative**, or the **second order derivative**, of a function *f* is the derivative of the derivative of *f*. Roughly speaking, the second derivative measures how the rate of change of a quantity is itself changing; for example, the second derivative of the position of an object with respect to time is the instantaneous acceleration of the object, or the rate at which the velocity of the object is changing with respect to time. In Leibniz notation:

The **triple product rule**, known variously as the **cyclic chain rule**, **cyclic relation**, **cyclical rule** or **Euler's chain rule**, is a formula which relates partial derivatives of three interdependent variables. The rule finds application in thermodynamics, where frequently three variables can be related by a function of the form *f*(*x*, *y*, *z*) = 0, so each variable is given as an implicit function of the other two variables. For example, an equation of state for a fluid relates temperature, pressure, and volume in this manner. The triple product rule for such interrelated variables *x*, *y*, and *z* comes from using a reciprocity relation on the result of the implicit function theorem and is given by

In differential calculus, there is no single uniform **notation for differentiation**. Instead, several different notations for the derivative of a function or variable have been proposed by different mathematicians. The usefulness of each notation varies with the context, and it is sometimes advantageous to use more than one notation in a given context. The most common notations for differentiation are listed below.

In mathematics, an **implicit curve** is a plane curve defined by an implicit equation relating two coordinate variables, commonly *x* and *y*. For example, the unit circle is defined by the implicit equation . In general, every implicit curve is defined by an equation of the form

In calculus, the **differential** represents the principal part of the change in a function *y* = *f*(*x*) with respect to changes in the independent variable. The differential *dy* is defined by

*Most of the terms listed in Wikipedia glossaries are already defined and explained within Wikipedia itself. However, glossaries like this one are useful for looking up, comparing and reviewing large numbers of terms together. You can help enhance this page by adding new terms or writing definitions for existing ones.*

- ↑ Leibniz, G., "Nova Methodus pro Maximis et Minimis",
*Acta Eruditorum*, Oct. 1684. - ↑ Euclid. "Euclid's Elements" . Retrieved 1 June 2015.
- 1 2 Shenk, Al. "e-CALCULUS Section 2.8" (PDF). p. 2.8. Retrieved 1 June 2015.
- ↑ Katz, Victor J. (2008).
*A History of Mathematics*(3rd ed.). Addison Wesley. p. 510. ISBN 978-0321387004. - ↑ Wolfson, Paul R. (2001). "The Crooked Made Straight: Roberval and Newton on Tangents".
*The American Mathematical Monthly*.**108**(3): 206–216. doi:10.2307/2695381. - ↑ Katz, Victor J. (2008).
*A History of Mathematics*(3rd ed.). Addison Wesley. pp. 512–514. ISBN 978-0321387004. - ↑ Noah Webster,
*American Dictionary of the English Language*(New York: S. Converse, 1828), vol. 2, p. 733, - ↑ Descartes, René (1954).
*The geometry of René Descartes*. Courier Dover. p. 95. ISBN 0-486-60068-8.External link in`|publisher=`

(help) - ↑ R. E. Langer (October 1937). "Rene Descartes".
*American Mathematical Monthly*. Mathematical Association of America.**44**(8): 495–512. doi:10.2307/2301226. JSTOR 2301226. - 1 2 Edwards Art. 191
- ↑ Strickland-Constable, Charles, "A simple method for finding tangents to polynomial graphs",
*Mathematical Gazette*, November 2005, 466–467. - 1 2 Edwards Art. 192
- ↑ Edwards Art. 193
- 1 2 Edwards Art. 196
- ↑ Edwards Art. 194
- ↑ Edwards Art. 195
- ↑ Edwards Art. 197
- ↑ Thomas, George B. Jr., and Finney, Ross L. (1979),
*Calculus and Analytic Geometry*, Addison Wesley Publ. Co.: p. 140. - ↑ Circles For Leaving Certificate Honours Mathematics by Thomas O’Sullivan 1997

- J. Edwards (1892).
*Differential Calculus*. London: MacMillan and Co. pp. 143 ff.

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*Encyclopedia of Mathematics*, EMS Press, 2001 [1994] - Weisstein, Eric W. "Tangent Line".
*MathWorld*. - Tangent to a circle With interactive animation
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