Tangent
1.Geometry
A line which touches a circle or ellipse at just one point. Below, the blue line is a tangent to the circle c.
Note the radius to the point of tangency is always perpendicular to the tangent line.
For more on this see Tangent to a circle.
2. Trigonometry
One of the trigonometry functions. In a right triangle, the tangent of an angle is the opposite side over the adjacent side.
For more on this see Trigonometry tangent function.
This page explores the derivatives of trigonometric functions in calculus. Interactive calculus applet.
Interactive demonstration of the graph of the tangent function in trigonometry
This page shows how to draw the two possible tangents to a given circle through an external point with compass and straightedge or ruler. This construction assumes you are already familiar with constructing the perpendicular bisector of a line segment.
Constructing the tangent to a circle at a given point on the circle with compass and straightedge or ruler. It works by using the fact that a tangent to a circle is perpendicular to the radius at the point of contact. It first creates a radius of the circle, then constructs the perpendicular bisector of the radius at the given point.
Introduction to the 6 trigonometry functions  sine, cosine, tangent, secant, cosecant, cotangent
A pictorial index to the parts of a circle. A diagram with links to full definitions.
A memory aid for remembering the definitions of sin, cos, and tan
Definition and properties of a tangent
Definition and properties of the tangent line to an ellipse
Definition of the tangent function as applied to right triangles in trigonometry
Definition of the arctan function in trigonometry. The inverse of the tangent function. The angle whose tangent is a given number.
Definition of the arctan function in trigonometry. The inverse of the tangent function. The angle whose tangent is a given number.
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