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### Course: Geometry (all content) > Unit 14

Lesson 9: Properties of tangents- Proof: Radius is perpendicular to tangent line
- Determining tangent lines: angles
- Determining tangent lines: lengths
- Proof: Segments tangent to circle from outside point are congruent
- Tangents of circles problem (example 1)
- Tangents of circles problem (example 2)
- Tangents of circles problem (example 3)
- Tangents of circles problems
- Challenge problems: radius & tangent
- Challenge problems: circumscribing shapes

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# Determining tangent lines: lengths

Solve two problems that apply properties of tangents to determine if a line is tangent to a circle.

## Problem 1

## Problem 2

## Want to join the conversation?

- how is angle AOC not a right angled triangle in problem 1(3 votes)
- The the first example is not a right triangle because it does not follow the Pythagorean Theorem of a^2 + b^2 = c^2. AC^2+OC^2 doesn't equal AO^2. (11^2 + 5^2 = 13^2, which turns out to be 146 = 169, not true). Both 45-45-90 and 30-60-90 triangles follow this rule.(19 votes)

- The first question is vague and doesn't explain how they found the length of AO. Where did y'all even get 8?(3 votes)
- 8 was given as the length of AB. BO is a radius of the circle and therefore has length of 5.

So, AO (which equals AB + BO) is 8 + 5 which is 13.

They also had to test whether there was a right angle at point C, so they used the Pythagorean theorem to see whether that was true. It wasn't, so in other words, AC wasn't perpendicular to OC, so therefore line AC was NOT tangent to circle O.(11 votes)

- what is the converse Pythagorean theorem?(2 votes)
- Normally we use the Pythagorean Theorem on a Right Triangle to find the length of a missing side measurement.

•The**Converse**Pythagorean Theorem is when**we already have the side lengths**of a triangle, but we…**don't know if it's a Right Triangle**

so it's when we**use the formula**through Comparison.*to check if a triangle has a 90° angle*

We must…

•Plug two of the triangle's side measurements into the corresponding variables of the Pythagorean Theorem.

•Calculate to see what the third side would be*if it were a Right Triangle*.

•Compare the triangle's given measurement to the calculated.

•If the corresponding values are equal then it's a Right Triangle and has a 90° angle.

If not, it is not.

So…

•To use the Pythagorean Theorem as its Converse,**is to apply the formula to**, and therefore is, or is not a Right Triangle.*prove or disprove*if a triangle has a 90° angle

(ㆁωㆁ) Hope this helps someone!(11 votes)

- Can someone explain why for problem two line BO is included in solving the problem while in problem 1 BO is left out? I understand that for problem 1 using the pythagorean theorem shows its not perpendicular but using that same method for problem 2 doesn't work and thus adding line BO is needed.(4 votes)
- You can find the length of BO in either question, using just the radius. This is what you use to find out if it is a right triangle and thus, you need BO. Hope this answers your question...(6 votes)

- Can someone explan #2 to me?

I dont understand because 2,3,4 is not a right triangle??(1 vote)- The sides of the triangle in problem 2 are 12, 16, and 20 (12+8), which does make it a right triangle, since 20² = 12²+16².(10 votes)

- how can we draw 2 common transverse tangents for 2 congruent circles if they have any distance between their centres?(3 votes)
- you dont that is something different you are using Pythagorean theorem here.(5 votes)

- why is the perpindicularity contradicting it is confusing because they look the same except for line length in both problems.(4 votes)
- Since we know all of the lengths in this triangle, we can check if Pythagorean theorem will agree with our assumption that these are right triangles.

Pythagorean theorem c² = a² + b²

Problem 1:

13² = 5² + 11²

169 = 25 + 121

169 ≠ 146 (These would be equal if we had 90° angle)

Problem 2:

20² = 12² + 16²

400 = 144 + 256

400 = 400 (This is a right triangle)(2 votes)

- how many types of tangent(4 votes)
- What does tangent mean again?(2 votes)
- A line is tangent to a circle when it touches the circle at exactly one point.(1 vote)

- the Pythagorean theorem is practically used everywhere.WHY?(2 votes)
- a^2+b^2=c^2 is used almost all the time for triangles.(1 vote)