Power of a Point Theorem

Introduction

The Power of a Point Theorem expresses the relation between the lengths involved with the intersection of two lines between each other and their intersections with a circle.

Theorem

There are three possibilities as displayed in the figure below.

The two lines are secants of the circle and intersect inside the circle (figure on the left). In this case, we have $AE\cdot CE = BE\cdot DE$ .
One of the lines is tangent to the circle while the other is a secant (middle figure). In this case, we have $AB^2 = BC\cdot BD$ .
Both lines are secants of the circle and intersect outside of it (figure on the right). In this case, we have $CB\cdot CA = CD\cdot CE.$

Alternate Formulation

This alternate formulation is much more compact, convenient, and general.

Consider a circle O and a point P in the plane where P is not on the circle. Now draw a line through P that intersects the circle in two places. The power of a point theorem says that the product of the the length from P to the first point of intersection and the length from P to the second point of intersection is constant for any choice of a line through P that intersects the circle. This constant is called the power of point P. For example, in the figure below

$PX^2 = PA_1\cdot PB_1 = PA_2\cdot PB_2 = \cdots = PA_i\cdot PB_i$

Notice how this definition still works if $A_k$ and $B_k$ coincide (as is the case with X). Consider also when P is inside the circle. The definition still holds in this case.

Additional Notes

One important result of this theorem is that both tangents from a point $P$ outside of a circle to that circle are equal in length.

Problems

The problems are divided into three categories: introductory, intermediate, and olympiad.

Introductory

Problem 1

Find the value of $x$ in the following diagram:

Solution

Problem 2

Find the value of $x$ in the following diagram.

Solution

Problem 3

(ARML) In a circle, chords $AB$ and $CD$ intersect at $R$ . If $AR:BR = 1:4$ and $CR:DR = 4:9$ , find the ratio $AB:CD.$

Solution

Problem 4

(ARML) Chords $AB$ and $CD$ of a given circle are perpendicular to each other and intersect at a right angle. Given that $BE = 16, DE = 4,$ and $AD = 5$ , find $CE$ .

Solution

Intermediate

Problem 1

Two tangents from an external point $P$ are drawn to a circle and intersect it at $A$ and $B$ . A third tangent meets the circle at $T$ , and the tangents $\overrightarrow{PA}$ and $\overrightarrow{PB}$ at points $Q$ and $R$ , respectively. Find the perimeter of $\triangle PQR$ .