Definition:Tangential Equation of Circle
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Definition
Let $\CC$ be a circle embedded in the Cartesian plane of radius $r$ with its center located at the origin.
Let $\LL$ be a straight line in the plane of $\CC$ whose equation is given by:
- $(1): \quad l x + m y + n = 0$
such that $l \ne 0$.
Then the equation:
- $\paren {l^2 + m^2} r^2 = n^2$
is known as the tangency equation of $\CC$.
Also see
- Condition of Tangency to Circle whose Center is Origin, which justifies the definition
Sources
- 1933: D.M.Y. Sommerville: Analytical Conics (3rd ed.) ... (previous) ... (next): Chapter $\text {III}$. The Circle: $13$. Condition that a straight line should touch a given circle