8.8.D
Use informal arguments to establish facts about the angle sum and exterior angle of triangles, the angles created when parallel lines are cut by a transversal, and the angle-angle criterion for similarity of triangles.
Khan Academy ResourcesAngle-angle triangle similarity criterionFinding angle measures using trianglesDetermine similar triangles: AnglesAngle relationships with parallel linesTriangle exterior angle property problemsFind angles in isosceles trianglesAngles, parallel lines, & transversalsTriangle angle challenge problemAngles in a triangle sum to 180° proofWorked example: Triangle angles (intersecting lines)Triangle angle challenge problem 2Triangle exterior angle exampleIsosceles & equilateral triangles problemsTriangle similarity postulates/criteriaIntro to triangle similarityParallel & perpendicular linesMissing angles with a transversal

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