# Triangle Proportionality

Using triangle proportionality theorem to prove parallel lines
Using triangle proportionality theorem to prove parallel lines

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Triangle Proportionality
Objectives: Use proportionality theorems to calculate segment lengths. To solve real-life problems, such as determining the dimensions of a piece of land.

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Use Proportionality Theorems
In this lesson, you will study four proportionality theorems. Similar triangles are used to prove each theorem.

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Activity In the diagram, DE | | AC.
Name a pair of similar triangles and explain why they are similar.

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Activity In the diagram, DE || AC.
Name a pair of similar triangles and explain why they are similar.

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10 8 6 Write a proportion & solve for x. 4. What is the ratio BD: DA? Reduce your answer. 5. What is the ratio BE: EC ? Reduce your answer. 6. What do you notice?

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

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Example 1 Find the length of XZ.

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

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Example 2 Determine whether PS || QR.

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Example 3 Find the value of x so that

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6.6 Use Proportionality Theorems
Discussion Recall that the distance between two parallel lines is always equal. This distance, however, must be measured along a perpendicular segment.

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Discussion But what if the distance is not perpendicular? Are these lengths still equal? Or does some other relationship exist?

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Proportionality Theorems!
Theorem 6.6 If three parallel lines intersect two transversals, then they divide the transversals proportionally.

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Example 4 Find the length of AB.

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Discussion Notice that the angle bisector also divides the third side of the triangle into two parts. Are those parts congruent? Or is there some other relationship between them? Recall that an angle bisector is a ray that divides an angle into two congruent parts.

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Proportionality Theorems!
Angle Bisector Proportionality Theorem If a ray bisects an angle of a triangle, then it divides the opposite side into segments whose lengths are proportional to the other two sides.

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Example 5 Find the value of x.

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Example 6 Find the value of x.

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