Understanding Secant Angles: A Guide to Their Types

Step-by-step Guide: Secant Angles and Their Types

Definition of a Secant:
A secant is a line that intersects a circle at two distinct points. When a secant segment is drawn from a point outside the circle, the part of the segment that lies outside the circle is often termed the “external segment,” while the part within the circle is the “internal segment.”

Types of Secant Angles:

• Angle Formed by Two Secants: An angle is formed at an external point by two secant lines intersecting outside the circle. The measure of this angle is half the difference of the measures of the intercepted arcs.
• Angle Formed by a Secant and a Tangent: When a secant and tangent line meet at an external point, the measure of the angle they form is half the measure of its intercepted arc.
• Angle Formed by Two Intersecting Chords: When two secants (in the form of chords) intersect inside a circle, the angle between them is half the sum of the measures of their intercepted arcs.

Properties of Secant Angles:

• Secant-Secant Power Theorem: For two secants drawn from an external point, the product of the lengths of one secant and its external segment equals the product of the lengths of the other secant and its external segment.
• Secant-Tangent Power Theorem: For a tangent and a secant drawn from a common external point, the square of the length of the tangent is equal to the product of the length of the secant and its external segment.

Examples

Example 1:
Two secants, \( AC \) and \( AB \), are drawn to a circle from an external point \( A \). If the intercepted arcs are \( 80^\circ \) and \( 140^\circ \) respectively, find the measure of angle \( CAB \).

Solution:
Using the formula for the angle formed by two secants:
\( \text{Angle} = \frac{\text{difference of intercepted arcs}}{2} \)
\( \angle CAB = \frac{140^\circ – 80^\circ}{2} = \frac{60^\circ}{2} = 30^\circ \)

Example 2:
A secant \( AB \) and a tangent \( AC \) meet outside the circle at point \( A \). If the intercepted arc by \( AB \) is \( 100^\circ \), determine the measure of angle \( CAA’ \).

Solution:
Using the formula for the angle formed by a secant and a tangent:
\( \text{Angle} = \frac{\text{intercepted arc}}{2} \)
\( \angle CAA’ = \frac{100^\circ}{2} = 50^\circ \)

Practice Questions:

1. Two secants meet outside a circle forming an angle of \( 50^\circ \). If one intercepted arc measures \( 120^\circ \), what’s the measure of the other intercepted arc?
2. A secant and a tangent meet outside a circle forming an angle of \( 40^\circ \). Determine the measure of the intercepted arc by the secant.
3. Two chords intersect inside a circle forming an angle of \( 35^\circ \). If one of the intercepted arcs measures \( 50^\circ \), determine the measure of the other intercepted arc.

Answers:

1. \( 180^\circ \)
2. \( 80^\circ \)
3. \( 40^\circ \)