8.G.5
Justify using informal arguments to establish facts about the angle sum of triangles (sum of the interior angles of a triangle is 180 degrees), measures of exterior angles of triangles, angles created when parallel lines are cut be a transversal (e.g., alternate interior angles), and angle-angle criterion for similarity of triangles.
Khan Academy ResourcesGetting ready for transformation propertiesTriangle angles reviewFinding angle measures between intersecting linesFinding angle measures using trianglesFind angles in trianglesAngle relationships with parallel linesEquation practice with anglesEquation practice with angle additionFind angles in isosceles trianglesAngles, parallel lines, & transversalsAngles in a triangle sum to 180° proofWorked example: Triangle angles (intersecting lines)Worked example: Triangle angles (diagram) Triangle exterior angle exampleIsosceles & equilateral triangles problemsMeasures of angles formed by a transversalMissing angles with a transversal

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