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# 9.11: Segments from Secants and Tangents

Difficulty Level: At Grade Created by: CK-12

What if you were given a circle with a tangent and a secant that intersect outside the circle? How could you use the length of some of the segments formed by their intersection to determine the lengths of the unknown segments? After completing this Concept, you'll be able to use the Tangent Secant Segment Theorem to solve problems like this one.

### Guidance

If a tangent and secant meet at a common point outside a circle, the segments created have a similar relationship to that of two secant rays. Recall that the product of the outer portion of a secant and the whole is equal to the same of the other secant. If one of these segments is a tangent, it will still be the product of the outer portion and the whole. However, for a tangent line, the outer portion and the whole are equal.

Tangent Secant Segment Theorem: If a tangent and a secant are drawn from a common point outside the circle (and the segments are labeled like the picture to the left), then $a^2=b(b+c)$ . This means that the product of the outside segment of the secant and the whole is equal to the square of the tangent segment.

#### Example A

Find the value of the missing segment.

Use the Tangent Secant Segment Theorem. Square the tangent and set it equal to the outer part times the whole secant.

$x^2=4(4+12)\\x^2=4 \cdot 16=64\\x=8$

#### Example B

Find the value of the missing segment.

Use the Tangent Secant Segment Theorem. Square the tangent and set it equal to the outer part times the whole secant.

$20^2=y(y+30)\\400=y^2+30y\\0=y^2+30y-400\\0=(y+40)(y-10)\\y=\xcancel{-40},10$

#### Example C

Fill in the blank and then solve for the missing segment.

$\underline{\;\;\;\;\;\;\;}=\underline{\;\;\;\;\;\;\;}(4+5)$

$x^2&=4(4+5)\\x^2&=36\\x&=6$

Watch this video for help with the Examples above.

### Guided Practice

Find $x$ in each diagram below. Simplify any radicals.

1.

2.

3.

Use the Tangent Secant Segment Theorem.

1.

$18^2&=10(10+x)\\324&=100+10x\\224&=10x\\x&=22.4$

2.

$x^2&=16(16+25)\\x^2&=656\\x&=4\sqrt{41}$

3.

$x^2&=24(24+20)\\x^2&=1056\\x&=4\sqrt{66}$

### Explore More

Solve for the missing segment.

Find $x$ in each diagram below. Simplify any radicals.

1. Describe and correct the error in finding $y$ . $10 \cdot 10&=y\cdot 15y\\100&=15y^2\\\frac{20}{3}&=y^2\\\frac{2\sqrt{15}}{3}&=y \quad {\color{red}\longleftarrow \ y} \ {\color{red}\text{is \underline{not} correct}}$

Solve for the unknown variable.

1. Find $x$ and $y$ .

Sep 24, 2012

Feb 26, 2015

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