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7.8: Triangle Proportionality

Difficulty Level: At Grade Created by: CK-12
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Triangle Proportionality 

Think about a midsegment of a triangle. A midsegment is parallel to one side of a triangle and divides the other two sides into congruent halves. The midsegment divides those two sides proportionally.

Investigation: Triangle Proportionality

Tools Needed: pencil, paper, ruler

  1. Draw ABC. Label the vertices.
  2. Draw XY¯¯¯¯¯¯¯¯ so that X is on AB¯¯¯¯¯¯¯¯ and Y is on BC¯¯¯¯¯¯¯¯. X and Y can be anywhere on these sides.
  3. Is XBYABC? Why or why not? Measure AX,XB,BY, and YC. Then set up the ratios AXXB and YCYB. Are they equal?
  4. Draw a second triangle, DEF. Label the vertices.
  5. Draw XY¯¯¯¯¯¯¯¯ so that X is on DE¯¯¯¯¯¯¯¯ and Y is on EF¯¯¯¯¯¯¯¯ AND XY¯¯¯¯¯¯¯¯ || DF¯¯¯¯¯¯¯¯.
  6. Is XEYDEF? Why or why not? Measure DX,XE,EY, and YF. Then set up the ratios DXXE and FYYE. Are they equal?

From this investigation, it is clear that if the line segments are parallel, then XY¯¯¯¯¯¯¯¯ divides the sides proportionally.

Triangle Proportionality Theorem: If a line parallel to one side of a triangle intersects the other two sides, then it divides those sides proportionally.

Triangle Proportionality Theorem Converse: If a line divides two sides of a triangle proportionally, then it is parallel to the third side.

Proof of the Triangle Proportionality Theorem:

Given: ABC with DE¯¯¯¯¯¯¯¯ || AC¯¯¯¯¯¯¯¯

Prove: ADDB=CEEB

Statement Reason
1. DE¯¯¯¯¯¯¯¯ || AC¯¯¯¯¯¯¯¯ Given
2. 12,34 Corresponding Angles Postulate
3. ABCDBE AA Similarity Postulate
4. AD+DB=AB,EC+EB=BC Segment Addition Postulate
5. ABBD=BCBE Corresponding sides in similar triangles are proportional
6. AD+DBBD=EC+EBBE Substitution PoE
7. ADBD+DBDB=ECBE+BEBE Separate the fractions
8. ADBD+1=ECBE+1 Substitution PoE (something over itself always equals 1)
9. ADBD=ECBE Subtraction PoE

Determining Ratios 

A triangle with its midsegment is drawn below. What is the ratio that the midsegment divides the sides into?

The midsegment’s endpoints are the midpoints of the two sides it connects. The midpoints split the sides evenly. Therefore, the ratio would be a:a or b:b. Both of these reduce to 1:1.

Solving for Unknown Lengths 

In the diagram below, EB¯¯¯¯¯¯¯¯ || CD¯¯¯¯¯¯¯¯. Find BC.

Use the Triangle Proportionality Theorem.

1015=BC1215(BC)BC=120=8

Determining if Two Lines are Parallel 

Is DE¯¯¯¯¯¯¯¯ || CB¯¯¯¯¯¯¯¯?

Use the Triangle Proportionality Converse. If the ratios are equal, then the lines are parallel.

618=13 and 824=13

Because the ratios are equal, DE¯¯¯¯¯¯¯¯ || CB¯¯¯¯¯¯¯¯.

Examples

The following Examples use the diagram below. DB¯¯¯¯¯¯¯¯FE¯¯¯¯¯¯¯¯.

Example 1

Name the similar triangles. Write the similarity statement.

DBCFEC

Example 2

BEEC=?FC

DF

Example 3

\begin{align*}\frac{EC}{CB} = \frac{CF}{?}\end{align*}

DC

Example 4

\begin{align*}\frac{DB}{?} = \frac{BC}{EC}\end{align*}

FE

Example 5

\begin{align*}\frac{FC+?}{FC} = \frac{?}{FE}\end{align*}

DF; DB

Review 

Use the diagram to answer questions 1-7. \begin{align*}\overline{AB} \ || \ \overline{DE}\end{align*}.

  1. Find \begin{align*}BD\end{align*}.
  2. Find \begin{align*}DC\end{align*}.
  3. Find \begin{align*}DE\end{align*}.
  4. Find \begin{align*}AC\end{align*}.
  5. What is \begin{align*}BD:DC\end{align*}?
  6. What is \begin{align*}DC:BC\end{align*}?
  7. We know that \begin{align*}\frac{BD}{DC}=\frac{AE}{EC}\end{align*} and \begin{align*}\frac{BA}{DE}=\frac{BC}{DC}\end{align*}. Why is \begin{align*}\frac{BA}{DE} \neq \frac{BD}{DC}\end{align*}?

Use the given lengths to determine if \begin{align*}\overline{AB} \ || \ \overline{DE}\end{align*}.

Find the unknown length.

  1. What is the ratio that the midsegment divides the sides into?

Review (Answers)

To view the Review answers, open this PDF file and look for section 7.8. 

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Vocabulary

TermDefinition
Congruent Congruent figures are identical in size, shape and measure.
midsegment A midsegment connects the midpoints of two sides of a triangle or the non-parallel sides of a trapezoid.
Parallel Two or more lines are parallel when they lie in the same plane and never intersect. These lines will always have the same slope.
Proportion A proportion is an equation that shows two equivalent ratios.
Triangle Proportionality Theorem The Triangle Proportionality Theorem states that if a line is parallel to one side of a triangle and it intersects the other two sides, then it divides those sides proportionally.
Triangle Proportionality Theorem Converse The Triangle Proportionality Theorem converse states that if a line divides two sides of a triangle proportionally, then it is parallel to the third side.

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Difficulty Level:
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Date Created:
Jul 17, 2012
Last Modified:
Aug 02, 2016
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MAT.GEO.542.L.2