(2) If the distance from the center of the circle to a straight line is equal to the radius of the circle, then this straight line is the tangent of the circle.

(3) If a straight line passes through the outer end of the radius and is perpendicular to the radius, then this straight line is the tangent of the circle.

Definition of tangent line

A tangent is a straight line that just touches a point on a curve. The methods to judge the tangent of a circle are as follows: a straight line with only one common point with the circle is the tangent of the circle; The straight line whose distance from the center of the circle is equal to the radius of the circle is the tangent of the circle; The straight line passing through the outer end of the radius and perpendicular to the radius is the tangent of the circle.

Main properties of tangent line

(1) Tangents and circles have only one common point.

(2) The distance between the tangent and the center of the circle is equal to the radius of the circle.

(3) The tangent is perpendicular to the radius passing through the tangent point.

(4) A straight line whose center is perpendicular to the tangent must pass through the tangent point.

(5) A straight line passing through the tangent point and perpendicular to the tangent order must pass through the center of the circle.

(6) The tangent and secant of a circle are drawn from a point outside the circle, and the length of the tangent is the middle term in the length ratio of the two lines from this point to the intersection of the secant and the circle.