Lesson plan

Prove and apply the conjecture that a line parallel to one side of a triangle divides the other two proportionally by using the properties of similarity

teaches Common Core State Standards CCSS.Math.Content.HSG-SRT.B.4 http://corestandards.org/Math/Content/HSG/SRT/B/4
teaches Common Core State Standards CCSS.Math.Practice.MP1 http://corestandards.org/Math/Practice/MP1
teaches Common Core State Standards CCSS.Math.Practice.MP3 http://corestandards.org/Math/Practice/MP3

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Big Ideas: We can use similarity to prove theorems about triangles. Parallel lines create segments that are proportional. The tasks in this lesson require students to show that segments of two sides are proportional if the line that creates them is parallel to the third side. Students will use prior knowledge of parallel lines, similarity, and the AA similarity criteria to prove the proportion. This lesson leads into solving problems involving similar triangles and right triangle trigonometry. Vocabulary: Similarity Proportionality