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IntroductionArchaeologists, among others, rely on the Angle-Angle (AA), Side-Angle-Side (SAS), and Side-Side-Side (SSS) similarity statements to determine actual distances and locations created by similar triangles. Many engineers, surveyors, and designers use these statements along with other properties of similar triangles in their daily work. Having the ability to determine if two triangles are similar allows us to solve many problems where it is necessary to find segment lengths of triangles.
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1.7.2: Working with Ratio Segments
Landscapers will often stake a sapling to strengthen the tree’s root system. A typical method of staking a tree is to tie wires to both sides of the tree and then stake the wires to the ground. If done properly, the two stakes will be the same distance from the tree.
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1.7.2: Working with Ratio Segments
1. Assuming the distance from
the tree trunk to each stake
is equal, what is the value of x?
2. How far is each stake from the tree?
3. Assuming the distance from the tree
trunk to each stake is equal, what is
the value of y?
4. What is the length of the wire from
each stake to the tie on the tree?
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1.7.2: Working with Ratio Segments
2y + 17
7y – 3
4x – 5 2x + 1
Key Concepts• If a line parallel to one side of a triangle intersects the
other two sides of the triangle, then the parallel line divides these two sides proportionally.
• This is known as the Triangle Proportionality Theorem.
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1.7.2: Working with Ratio Segments
Key Concepts, continued
• In the figure above, ; therefore, .5
1.7.2: Working with Ratio Segments
Theorem
Triangle Proportionality TheoremIf a line parallel to one side of a triangle intersects the other two sides of the triangle, then the parallel line divides these two sides proportionally.
Key Concepts, continued
• In the figure above, ; therefore, . 6
1.7.2: Working with Ratio Segments
Guided Practice
Example 1Find the length of .
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1.7.2: Working with Ratio Segments
Guided Practice: Example 1, continued
The length of is 8.25 units.8
1.7.2: Working with Ratio Segments
✔
Create a proportion.
Substitute the known lengths of each segment.
(3)(5.5) = (2)(x) Find the cross products.
16.5 = 2x Solve for x.
x = 8.25
Guided Practice
Example 4Is ?
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1.7.2: Working with Ratio Segments
Guided Practice: Example 4, continued
Determine if divides and proportionally.
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1.7.2: Working with Ratio Segments
therefore, they are not parallel because of the Triangle Proportionality Theorem
Key Concepts, continued• This is helpful when determining if two lines or
segments are parallel.
• It is possible to determine the lengths of the sides of triangles because of the Segment Addition Postulate.
• This postulate states that if B is between A and C, then AB + BC = AC.
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1.7.2: Working with Ratio Segments
Key Concepts, continued• It is also true that if AB + BC = AC, then B is between A and C.
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1.7.2: Working with Ratio Segments
Key Concepts, continued• Segment congruence is also helpful when determining
the lengths of sides of triangles.
• The Reflexive Property of Congruent Segments means that a segment is congruent to itself, so
• According to the Symmetric Property of Congruent
Segments, if , then .
• The Transitive Property of Congruent Segments
allows that if and , then .13
1.7.2: Working with Ratio Segments
Key Concepts, continued• This information is also helpful when determining
segment lengths and proving statements.
• If one angle of a triangle is bisected, or cut in half, then the angle bisector of the triangle divides the opposite side of the triangle into two segments that are proportional to the other two sides of the triangle.
• This is known as the Triangle Angle Bisector Theorem.
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1.7.2: Working with Ratio Segments
Key Concepts, continued
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1.7.2: Working with Ratio Segments
Theorem
Triangle Angle Bisector TheoremIf one angle of a triangle is bisected, or cut in half,then the angle bisectorof the triangle divides the opposite side of the triangle into two segments that are proportional to the other two sides of the triangle.
