Big Ideas Math Geometry, 2014
BI
Big Ideas Math Geometry, 2014 View details
1. Lines and Segments That Intersect Circles
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Exercise 46 Page 536

Begin by drawing PR, PT, and PS. Then consider the Hypotenuse-Leg Theorem.

See solution.

Practice makes perfect

We want to prove the External Tangent Congruence Theorem.

External Tangent Congruence Theorem

Tangent segments from a common external point are congruent.

We will prove that SR ≅ ST, given that SR and ST are tangent to ⊙ P.

To do so, we will use congruent triangles. Let's start by drawing PR, PT, and PS.

Having drawn these segments, we have two triangles, △ SRP and △ STP. From here, we will continue by showing that these triangles are congruent. As we can see, PR and PT are the radii of ⊙ P, and R and T are their corresponding points of tangency. Let's consider the Tangent Line to Circle Theorem.

Tangent Line to Circle Theorem

In a plane, a line is tangent to a circle if and only if the line is perpendicular to a radius of the circle at its endpoint on the circle.

With this theorem, we can say that ∠ PRS and ∠ PTS are right angles.

Therefore, we can conclude that △ SRP and △ STP are right triangles. Next, recall that radii of a circle are congruent. Therefore, PR and PT are congruent segments.

We can also see that △ SRP and △ STP share the same hypotenuse PS. By the Reflexive Property of Congruence, PS is congruent to itself.

Combining all this information, we can say that the hypotenuse and one leg of △ SRP are congruent to the hypotenuse and one leg of △ STP. Therefore, by the Hypotenuse-Leg Theorem, we have that △ SRP and △ STP are congruent triangles. △ SRP ≅ △ STP Finally, because corresponding parts of congruent triangles are congruent, we can conclude that SR≅ ST.

Let's summarize the above process in a flow proof.