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Mathematics LibreTexts

1.A: Angle Measures

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    The purpose of this lesson is to understand basic angle relationships such as complementary, supplementary, and vertical angles.

    This lesson will address the following CCRS Standard(s) for Geometry:

    • 8.G.5: Use informal arguments to establish facts about the angle sum and exterior angles of triangles, above 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
    • G.CO.1: Know precise definitions of angle, circle, perpendicular line, parallel line, and line segment, based on the undefined notions of point, line, distance along a line, and distance around a circular arc


    1. Take notes while watching videos below
    2. Go to and log into our course to complete assignment 1.A with 80% or better.


    Complete assignment 1.A with 80% or better at


    In this lesson we have learned:

    • Angles are measured between 0o and 1800 where angles less than 900 are called acute angles, angles that are 900 are called right angles, and angles that are more than 900 are called obtuse angles.
    • Complementary angles are two angles which add to 900
    • Supplementary angles are two angles which add to 1800, they are often found together (adjacent) forming a straight line.
    • Vertical angles are made with two intersecting lines where opposite angles are congruent or have the same measure.
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