Tangent (geometry)

Revision as of 19:06, 28 November 2007 by Azjps (talk | contribs) (Tangent line moved to Tangent (geometry): A tangent is a line)

A tangent line is a linear approximate to a curve. That is, if you zoom in very closely, the tangent line and the curve will become indistinguishable from each other at the point in which they intersect.

Intersection

Locally, a tangent line intersects a curve in a single point. However, if a curve is neither convex nor concave, it is possible for a tangent line to intersect a curve in additional points. For instance, the tangent line of the curve $y = \sin x$ at $(0, 0)$ intersects it in 1 point, while the tangent line at $\left(\frac{\pi}4, \frac{1}{\sqrt 2}\right)$ intersects it in 2 points and the tangent line at $\left(\frac{\pi}2, 1\right)$ intersects it in infinitely many points (and is in fact the tangent line at each point of intersection).

At a given point, a curve may have either 0 or 1 tangent lines. For the graph of a function, the condition "having a tangent line at a point" is equivalent to "being a differentiable function at that point." It is a fairly strong condition on a function -- only continuous functions may have tangent lines, and there are many continuous functions which fail to have tangent lines either at some points (for instance, the absolute value function $y = |x|$ at $x = 0$) or even at all points!

This article is a stub. Help us out by expanding it.

See also