M.8.20

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. (e.g., 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 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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