Tangent line to a circle

Definition

Let $C$ be a circle in the Euclidean plane $\mathbb{R}^2$, and let $T$ be a point of $C$. A line $\ell$ is tangent to $C$ at $T$ if $\ell \cap C = \{T\}$. The point $T$ is the point of tangency.

Remarks

A line meets a circle in two points, in one point or in no point. A line with two common points is a secant line. For the circle $(x - a)^2 + (y - b)^2 = r^2$ and a point $(x_1, y_1)$ on it, the tangent at this point has the equation $(x - x_1)(x_1 - a) + (y - y_1)(y_1 - b) = 0$. The tangent at $T$ is the line through $T$ perpendicular to the radius to $T$. See a tangent is perpendicular to the radius. From a point outside the circle there are exactly two tangent lines to the circle.

Explanations

L1 human

A tangent line just touches a circle at one point and stays outside the circle. For the unit circle, the line $y = 1$ is tangent at $(0, 1)$. The line $y = 0$ is not tangent: it meets the circle at $(1, 0)$ and at $(-1, 0)$.