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Inscribed Similar Triangles

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What if you drew a line from the right angle of a right triangle perpendicular to the side that is opposite that angle? How could you determine the length of that line? After completing this Concept, you'll be able to solve problems like this one.

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CK-12 Foundation: Inscribed Similar Triangles

Guidance

Remember that if two objects are similar, their corresponding angles are congruent and their sides are proportional in length. The altitude of a right triangle creates similar triangles.

Inscribed Similar Triangles Theorem: If an altitude is drawn from the right angle of any right triangle, then the two triangles formed are similar to the original triangle and all three triangles are similar to each other.

In \triangle ADB, m \angle A = 90^{\circ} and \overline{AC} \bot \overline{DB} :

So, \triangle ADB \sim \triangle CDA \sim \triangle CAB :

This means that all of the corresponding sides are proportional. You can use this fact to find missing lengths in right triangles.

Example A

Find the value of x .

Separate the triangles to find the corresponding sides.

Set up a proportion.

\frac{\text{shorter leg in} \ \triangle EDG}{\text{shorter leg in} \ \triangle DFG} &= \frac{\text{hypotenuse in} \ \triangle EDG}{\text{hypotenuse in} \ \triangle DFG}\\\frac{6}{x} &= \frac{10}{8}\\48 &= 10x\\4.8 &= x

Example B

Find the value of x .

Set up a proportion.

\frac{shorter \ leg \ of \ smallest \ \triangle}{shorter \ leg \ of \ middle \ \triangle} &= \frac{longer \ leg \ of \ smallest \ \triangle}{longer \ leg \ of \ middle \ \triangle}\\\frac{9}{x} &= \frac{x}{27}\\x^2 &= 243\\x &= \sqrt{243} = 9 \sqrt{3}

Example C

Find the values of x and y .

Separate the triangles. Write a proportion for x .

\frac{20}{x} &= \frac{x}{35}\\		x^2 &= 20 \cdot 35\\x &= \sqrt{20 \cdot 35}\\x &= 10 \sqrt{7}

Set up a proportion for y . Or, now that you know the value of x you can use the Pythagorean Theorem to solve for y . Use the method you feel most comfortable with.

\frac{15}{y} &= \frac{y}{35} && (10 \sqrt{7})^2 + y^2 = 35^2\\y^2 &= 15 \cdot 35 && \qquad 700 + y^2 =1225\\y &= \sqrt{15 \cdot 35} && \qquad \qquad \quad \ y = \sqrt{525} = 5 \sqrt{21}\\y &= 5 \sqrt{21}

CK-12 Foundation: Inscribed Similar Triangles

Guided Practice

1. Find the value of x .

2. Now find the value of y in \triangle RST above.

3. Write the similarity statement for the right triangles in the diagram.

Answers:

1. Set up a proportion.

\frac{\text{shorter leg in} \ \triangle SVT}{\text{shorter leg in} \ \triangle RST} &= \frac{\text{hypotenuse in} \ \triangle SVT}{\text{hypotenuse in} \ \triangle RST}\\\frac{4}{x} &= \frac{x}{20}\\x^2 &= 80\\x &= \sqrt{80} = 4 \sqrt{5}

2. Use the Pythagorean Theorem.

y^2 + \left( 4 \sqrt{5} \right )^2 &= 20^2\\y^2 + 80 &= 400\\y^2 &= 320\\y &= \sqrt{320} = 8 \sqrt{5}

3. \triangle ACD \sim \triangle DCB \sim \triangle ADB

Practice

Fill in the blanks.

  1. \triangle BAD \sim \triangle \underline{\;\;\;\;\;\;\;\;\;} \sim \triangle \underline{\;\;\;\;\;\;\;\;\;}
  2. \frac{BC}{?} = \frac{?}{CD}
  3. \frac{BC}{AB} = \frac{AB}{?}
  4. \frac{?}{AD} = \frac{AD}{BD}

Write the similarity statement for the right triangles in each diagram.

Use the diagram to answer questions 7-10.

  1. Write the similarity statement for the three triangles in the diagram.
  2. If JM = 12 and ML = 9 , find KM .
  3. Find JK .
  4. Find KL .

Find the length of the missing variable(s). Simplify all radicals.

  1. Fill in the blanks of the proof for the Inscribed Similar Triangles Theorem.

Given : \triangle ABD with \overline{AC} \perp \overline {DB} and \angle DAB is a right angle.

Prove : \triangle ABD \sim \triangle CBA \sim \triangle CAD

Statement Reason
1. 1. Given
2. \angle DCA and \angle ACB are right angles 2.
3. \angle DAB \cong \angle DCA \cong \angle ACB 3.
4. 4. Reflexive PoC
5. 5. AA Similarity Postulate
6. B \cong \angle B 6.
7. \triangle CBA \cong \triangle ABD 7.
8. \triangle CAD \cong \triangle CBA 8.

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