options: Operations & Algebra: students learn to solve problems using algebraic thinking and operations such as addition, subtraction, multiplication, and division. They may learn to identify and explain arithmetic patterns, evaluate or manipulate numerical expressions, and reason with equations and inequalities.

Number Systems and Quantity: students learn to understand the complex number system and reason quantitatively.

Measurement & Data: students learn to work with data and measure attributes such as time, money, length, area, and volume. They may learn to compare measurements to operations and convert between different units of measure.

Counting & Cardinality: students learn to know number names and the count sequence, count to tell the number of objects, and compare numbers.

Ratios & Proportional Relationships: students learn to recognize, describe, represent, and reason with ratios, rates, proportional relationships, and percent.

Statistics & Probability: students learn to analyze and produce data distributions, and build understanding of univariate and bivariate data. They learn to interpret data, make statistical inferences, justify conclusions, understand rules of probability, and use probability to make decisions.

Number & Operations in Base Ten: students learn to work with the base-ten system and build place value understanding.

Functions: students learn to define, use, and evaluate functions to model relationships between quantities.

Geometry: students learn to classify geometric figures by their properties, and understand and compare the relationships between them. Students develop and use formulas to compute lengths, areas, and volumes, and they use transformations to generate new shapes and compare existing ones. Geometry can be studied with and without coordinates.

Number & Operations - Fractions: students learn to understand fractions as numbers and may work with them using addition, subtraction, multiplication, and division.

Modeling: students learn to choose and use appropriate mathematics and statistics to analyze real-world empirical situations, improve decisions, and report on their conclusions and the reasoning behind them.
problem_activity: Activity
In this activity, students calculate side lengths of similar triangles. They can use the scale factor between the similar triangles, studied in depth in previous lessons. Or they can look at internal ratios between corresponding side lengths within the triangles, introduced in the previous lesson. Students need to think strategically about which side lengths to calculate first since there are many missing values. As they discover more side lengths, this opens up more paths for finding the remaining values.
As students work, monitor for students who:
Use scale factors between triangles.
Notice that the long side is twice the short side in
\(GHI\)
and use that to find
\(c\)
,
\(d\)
, or
\(e\)
.
Notice that the long sides are equal in
\(ABC\)
and use that to find
\(h\)
,
\(d\)
, or
\(e\)
.
Select students who use different strategies to find side lengths to share during the discussion.
Launch
Tell students, “There are many ways to find the values of the unknown side lengths in similar triangles. Use what you have learned so far.” Give students 5 minutes of quiet work time followed by 5 minutes small group discussion and then a whole-class discussion.
Representation: Internalize Comprehension.
Demonstrate and encourage students to use color coding and annotations to highlight connections between representations in a problem. For example, use the same color for corresponding side lengths.
Supports accessibility for: Visual-spatial processing
Student Facing
Triangles
\(ABC\)
,
\(EFD\)
, and
\(GHI\)
are all similar. The side lengths of the triangles all have the same units. Find the unknown side lengths.
[IMG]
Student Response
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Anticipated Misconceptions
If students have trouble locating corresponding sides, suggest that they use tracing paper so they can rotate and or translate them. Another technique is to color corresponding side lengths the same color. For example, they could color
\(AB\)
,
\(EF\)
, and
\(GH\)
all red.
Activity Synthesis
Ask selected students to share the following strategies:
using (external) scale factors to move from one triangle to another
using quotients of corresponding side lengths within the triangles (internal scale factors)
Both methods are efficient and the method to use is guided by what information is missing and the numbers involved in the calculations. For example, if
\(h\)
is the first missing value we find, then comparing with triangle
\(ABC\)
and using internal scale factors is appropriate. To find
\(c\)
, again we can compare
\(ABC\)
and
\(GHI\)
and internal scale factors are appropriate again because
\(\frac{6}{5}\)
is half of
\(\frac{12}{5}\)
whereas comparing
\(\frac{12}{5}\)
and
\(4\)
is more involved (the scale factor is
\(\frac{5}{3}\)
from
\(\triangle GHI\)
to
\(\triangle ABC\)
).
Ask students to articulate how they knew which sides of the similar triangles correspond. Make sure to make the following reasoning pathways explicit for all:
Triangle
\(ABC\)
has two equal side lengths, so the other two triangles will as well. This insight is efficient for finding
\(h\)
.
One side of triangle
\(GHI\)
is twice the length of another side, so this will be true of the other triangles as well. This insight is helpful for finding
\(c\)
,
\(d\)
, and
\(e\)
.
Emphasize that there are many different relationships that can be used to find side lengths of similar triangles.
Speaking: MLR8 Discussion Supports:
To support students to articulate how they knew which sides of the similar triangles correspond, provide sentence frames such as, “For similar triangles _____ and _____ I know that sides _____ and _____ correspond because ________." Sentence frames invite and incentivize more student participation, conversation, and meta-awareness of language. This routine should help students reason about the ratio of corresponding sides of similar triangles and communicate their understanding.
Design Principle(s): Support sense-making, Optimize output (for comparison)


correct_option_index: 8