The way to prove that a straight line is tangent to a circle is to prove that the angle between the center of the circle and the tangent is 90, and then to prove that the angle is equal, and then to prove that it is an equilateral triangle, or other problems related to angle proof such as parallel lines.

If we know that tangent is often used to calculate the length of line segments, tangent length theorem is often used, of course, there will be tangent theorem.