standard_description: 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.
problem_activity: Warm-up
Polygons are special shapes because once we know the vertices, listed in order, we can join them by line segments to produce the polygon. This is important when performing rigid transformations. Because rigid transformations take line segments to line segments, once we track where the vertices of a polygon go, we can join them in the correct order with segments to find the image of the polygon.
In this warm-up, students begin to explore this structure, finding corresponding points in congruent polygons which are
not
vertices.
Launch
Give 3 minutes quiet think time followed by 2 minutes for a whole-class discussion.
Student Facing
Trapezoids
\(ABCD\)
and
\(A’B’C’D’\)
are congruent.
Draw and label the points on
\(A’B’C’D’\)
that correspond to
\(E\)
and
\(F\)
.
Draw and label the points on
\(ABCD\)
that correspond to
\(G’\)
and
\(H’\)
.
Draw and label at least three more pairs of corresponding points.
[IMG]
Student Response
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Anticipated Misconceptions
Students may struggle to find corresponding points that are not vertices. Suggest that they use tracing paper or the structure of the grid to help identify these corresponding points.
Activity Synthesis
Remind students that when two figures are congruent, every point on one figure has a corresponding point on the other figure.
Ask students what methods they used to find their corresponding points. Possible answers include:
Using the grid and the corresponding vertices to keep track of distances and place the corresponding point in the right place
Using tracing paper and rigid motions taking one polygon to the other


aligns: no