Lesson 9
Composing Figures
Let’s use reasoning about rigid transformations to find measurements without measuring.
9.1: Angles of an Isosceles Triangle
Here is a triangle.
 Reflect triangle \(ABC\) over line \(AB\). Label the image of \(C\) as \(C’\).
 Rotate triangle \(ABC’\) around \(A\) so that \(C’\) matches up with \(B\).

What can you say about the measures of angles \(B\) and \(C\)?
9.2: Triangle Plus One
Here is triangle \(ABC\).

Draw midpoint \(M\) of side \(AC\).

Rotate triangle \(ABC\) 180 degrees using center \(M\) to form triangle \(CDA\). Draw and label this triangle.

What kind of quadrilateral is \(ABCD\)? Explain how you know.
In the activity, we made a parallelogram by taking a triangle and its image under a 180degree rotation around the midpoint of a side. This picture helps you justify a wellknown formula for the area of a triangle. What is the formula and how does the figure help justify it?
9.3: Triangle Plus Two
The picture shows 3 triangles. Triangle 2 and Triangle 3 are images of Triangle 1 under rigid transformations.

Describe a rigid transformation that takes Triangle 1 to Triangle 2. What points in Triangle 2 correspond to points \(A\), \(B\), and \(C\) in the original triangle?

Describe a rigid transformation that takes Triangle 1 to Triangle 3. What points in Triangle 3 correspond to points \(A\), \(B\), and \(C\) in the original triangle?

Find two pairs of line segments in the diagram that are the same length, and explain how you know they are the same length.

Find two pairs of angles in the diagram that have the same measure, and explain how you know they have the same measure.
9.4: Triangle ONE Plus
Here is isosceles triangle \(ONE\). Its sides \(ON\) and \(OE\) have equal lengths. Angle \(O\) is 30 degrees. The length of \(ON\) is 5 units.

Reflect triangle \(ONE\) across segment \(ON\). Label the new vertex \(M\).

What is the measure of angle \(MON\)?

What is the measure of angle \(MOE\)?

Reflect triangle \(MON\) across segment \(OM\). Label the point that corresponds to \(N\) as \(T\).

How long is \(\overline{OT}\)? How do you know?

What is the measure of angle \(TOE\)?

If you continue to reflect each new triangle this way to make a pattern, what will the pattern look like?
Summary
Earlier, we learned that if we apply a sequence of rigid transformations to a figure, then corresponding sides have equal length and corresponding angles have equal measure. These facts let us figure out things without having to measure them!
For example, here is triangle \(ABC\).
We can reflect triangle \(ABC\) across side \(AC\) to form a new triangle:
Because points \(A\) and \(C\) are on the line of reflection, they do not move. So the image of triangle \(ABC\) is \(AB'C\). We also know that:
 Angle \(B'AC\) measures \(36^\circ\) because it is the image of angle \(BAC\).
 Segment \(AB'\) has the same length as segment \(AB\).
When we construct figures using copies of a figure made with rigid transformations, we know that the measures of the images of segments and angles will be equal to the measures of the original segments and angles.
Glossary Entries
 corresponding
When part of an original figure matches up with part of a copy, we call them corresponding parts. These could be points, segments, angles, or distances.
For example, point \(B\) in the first triangle corresponds to point \(E\) in the second triangle. Segment \(AC\) corresponds to segment \(DF\).
 rigid transformation
A rigid transformation is a move that does not change any measurements of a figure. Translations, rotations, and reflections are rigid transformations, as is any sequence of these.
 vertical angles
Vertical angles are opposite angles that share the same vertex. They are formed by a pair of intersecting lines. Their angle measures are equal.
For example, angles \(AEC\) and \(DEB\) are vertical angles. If angle \(AEC\) measures \(120^\circ\), then angle \(DEB\) must also measure \(120^\circ\).
Angles \(AED\) and \(BEC\) are another pair of vertical angles.