8.G.5: Use informal arguments to establish facts about the angle sum and exterior angle of triangles, about the angles created when parallel lines are cut by a transversal, and the angle-angle criterion for similarity of triangles. For example, arrange three copies of the same triangle so that the sum of the three angles appears to form a line, and give an argument in terms of transversals why this is so.

Use informal arguments to establish facts about the angle sum and exterior angle of triangles, about the angles created when parallel lines are cut by a transversal, and the angle-angle criterion for similarity of triangles. For example, arrange three copies of the same triangle so that the sum of the three angles appears to form a line, and give an argument in terms of transversals why this is so.

Khan Academy Resources
Getting ready for transformation propertiesTriangle angles reviewFinding angle measures between intersecting linesFinding angle measures using trianglesFind angles in trianglesVertical anglesAngle relationships with parallel linesEquation practice with anglesEquation practice with angle additionCreate equations to solve for missing anglesFind angles in isosceles trianglesFinding missing anglesAngles, parallel lines, & transversalsVertical anglesAngles in a triangle sum to 180° proofWorked example: Triangle angles (intersecting lines)Worked example: Triangle angles (diagram) Triangle exterior angle exampleIsosceles & equilateral triangles problemsEquation practice with supplementary anglesEquation practice with vertical anglesFind measure of angles word problemMeasures of angles formed by a transversalFind measure of vertical anglesMissing angles with a transversal
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