Informal Proof Of Angle-Angle Criterion

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Similar Triangles
Lesson Plans and Worksheets for Grade 8
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More Lessons for Grade 8
Common Core For Grade 8

New York State Common Core Math Grade 8, Module 3, Lesson 10

Worksheets for Grade 8

Lesson 10 Student Outcomes

• Students know an informal proof of the Angle-Angle (AA) criterion for similar triangles.
• Students present informal arguments as to whether or not triangles are similar based on Angle-Angle criterion.

Lesson 10 Summary

Two triangles are said to be similar if they have two pairs of corresponding angles that are equal.

Classwork

Concept Development
Exercises

1. Use a protractor to draw a pair of triangles with two pairs of equal angles. Then measure the lengths of sides, and verify that the lengths of their corresponding sides are equal in ratio.
2. Draw a new pair of triangles with two pairs of equal angles. Then measure the lengths of sides, and verify that the lengths of their corresponding sides are equal in ratio.

Examples 1 - 3
Are the triangles shown below similar? Present an informal argument as to why they are or why they are not.

Exercises 3 - 5
Are the triangles shown below similar? Present an informal argument as to why they are or why they are not.

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