Radius & Tangent (Edexcel A-Level Mathematics): Flashcards

A tangent touches the circle at exactly one point.

A normal is at right angles to a tangent.

Distance from centre to any point on the circumference.

The radius is $r$, and center is at $(h, k)$.

The radius is perpendicular to the tangent at contact.

$x_1x + y_1y = r^2$.

Tangents from an external point are equal in length.

Set the discriminant of the resulting quadratic to zero.

$k$ must be 3 or 11 for tangents to exist.

Its equation is $x^2 + y^2 = r^2$.