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Lesson Plan

Determine that the sum of the interior angles of a triangle is 180 degrees by giving an informal argument

teaches Common Core State Standards CCSS.Math.Content.8.G.A.5
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Big Ideas: The three angles in a triangle have a sum of 180 degrees. Equivalence of angles can be represented in many ways. In this lesson, students will manipulate angles of a triangle to determine that all three angles, when placed adjacent to each other, will form a straight line and, therefore, 180 degrees. Students will then apply their knowledge about the sum of the interior angles of a triangle to determine the measure of missing angles. Vocabulary: triangle angle sum, complementary, supplementary, isosceles, equilateral, right triangle, obtuse, adjacent Special Materials: Three congruent triangles cut out for each student. (or Patty Paper to trace 3 copies of the same triangle)
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