standard_description: Derive the formula A = 1/2 ab sin(C) for the area of a triangle by drawing an auxiliary line from a vertex perpendicular to the opposite side.
problem_activity: Activity
The purpose of this activity is to relate the process for
building
a triangle given 3 side lengths (using cardboard strips and metal fasteners) to the process for
drawing
a triangle given 3 side lengths (using a compass). Students use the cardboard strips as an informal compass for drawing all the possible locations where the given segments could end. They are reminded of their work with circles in a previous unit: that a circle is the set of all the points that are equally distant from a center point and that a compass is a useful tool, not just for drawing circles, but also for transferring lengths in general. This prepares them for using a compass to draw triangles in future lessons.
In this activity, students also consider what their drawing would look like if the two shorter sides were too short to make a triangle with the third given side length.
Left-handed students may find it easier to start with drawing the 3-inch circle on the left side of the 4-inch segment.
Digital
Launch
Arrange students in groups of 2.
[IMG]
Explain that students will be using the Trace feature to see the path of the point. It is accessed by right-clicking on the point.
Action and Expression: Internalize Executive Functions.
Begin with a small-group or whole-class demonstration of how to use the cardboard strips as an informal compass for drawing all the possible locations where the given segments could end.
Supports accessibility for: Memory; Conceptual processing
Student Facing
We'll explore a method for drawing a triangle that has three specific side lengths.  Use the applet to answer the questions.
Follow these instructions to mark the possible endpoints of one side:
For now, ignore segment
\(AC\)
, the 3-inch side length on the left side
[IMG]
Let segment
\(BD\)
be the 3-unit side length on the right side.  Right-click on point
\(D\)
, check Trace On.  Rotate the point, drawing all the places where a 3-inch side could end.
What shape have you drawn while moving
\(BD\)
around?  Why?  Which tool in your geometry toolkit can do something similiar?
Use your drawing to create two unique triangles, each with a base of length 4 inches and a side of length 3 inches.  Use a different color to draw each triangle.
Repeat the previous instructions, letting segment
\(AC\)
be the 3-unit side length.
Using a third color, draw a point where the two traces intersect. Using this third color, draw a triangle with side lengths of 4 inches, 3 inches, and 3 inches.
Student Response
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for free access to Student Response.
Print
Launch
Arrange students in groups of 2. Distribute one copy of the blackline master to each group. Make sure each group has one complete set of strips and fasteners from the previous activity. Provide access to geometry toolkits and compasses.
Tell students to take one 4-inch piece and two 3-inch pieces and connect them so that the 4-inch piece is in between the 3-inch pieces as seen in the image. If necessary, display the image for all to see. Students should not connect the 3-inch pieces to each other.
[IMG]
Explain to students that the sheet distributed to them is the 4-inch segment that is mentioned in the task statement and they will be drawing on that sheet.
[IMG]
If using the digital version of the activity, students will be using the Trace feature  to see the path of the point. It is accessed by right-clicking on the point.
Action and Expression: Internalize Executive Functions.
Begin with a small-group or whole-class demonstration of how to use the cardboard strips as an informal compass for drawing all the possible locations where the given segments could end.
Supports accessibility for: Memory; Conceptual processing
Student Facing
We’ll explore a method for drawing a triangle that has three specific side lengths. Your teacher will give you a piece of paper showing a 4-inch segment as well as some instructions for which strips to use and how to connect them.
Follow these instructions to mark the possible endpoints of one side:
Put your 4-inch strip directly on top of the 4-inch segment on the piece of paper. Hold it in place.
For now, ignore the 3-inch strip on the left side. Rotate it so that it is out of the way.
In the 3-inch strip on the
right
side, put the tip of your pencil in the hole on the end that is not connected to anything. Use the pencil to move the strip around its hinge, drawing all the places where a 3-inch side could end.
Remove the connected strips from your paper.
What shape have you drawn while moving the 3-inch strip around? Why? Which tool in your geometry toolkit can do something similar?
Use your drawing to create two unique triangles, each with a base of length 4 inches and a side of length 3 inches. Use a different color to draw each triangle.
Reposition the strips on the paper so that the 4-inch strip is on top of the 4-inch segment again. In the 3-inch strip on the
left
side, put the tip of your pencil in the hole on the end that is not connected to anything. Use the pencil to move the strip around its hinge, drawing all the places where another 3-inch side could end.
Using a third color, draw a point where the two marks intersect. Using this third color, draw a triangle with side lengths of 4 inches, 3 inches, and 3 inches.
Student Response
Teachers with a valid work email address can
click here to register or sign in
for free access to Student Response.
Activity Synthesis
Display a 4-inch strip connected to two 3-inch strips, positioned parallel to each other as pictured in the Launch. To help students connect the process of building with cardboard strips to drawing on paper, ask questions like:
“If you want to
build
a triangle with these side lengths, how do you know at what angle to position the cardboard strips?” (Turn the sides until their unattached endpoints are touching.)
“If you want to
draw
a triangle with these side length, how can you know at what angle to draw the sides?” (Find the point where both circles intersect.)
“We have seen with the cardboard strips that an unknown angle works like a hinge. How is that represented in your drawing?” (with a circle centered on the endpoint of one segment and a radius the length of the other segment)
Select students to share their drawings with the class. To reinforce the patterns that students noticed in the previous activity, consider asking questions like these:
“How many different triangles could we draw when we had only traced a circle on one side? Why?” (Lots of different triangles, because we were only using two of the given side lengths.)
“What is the longest the third side could have been? And the shortest?” (Less the 7 inches; More than 1 inch)
“How many different triangles could we draw once we had traced a circle on each side?” (It looked like there were 2 different triangles, but they are identical copies, so there’s really only 1 unique triangle.)
Speaking, Listening, Representing: MLR7 Compare and Connect.
Use this routine to help students compare their processes for building and drawing their triangles. Ask students “What is the same and what is different?” about the approaches. Draw students' attention to the connections between building and drawing such as: opening the hinge between the cardboard strips and drawing the circle using the compass. These exchanges strengthen students' language use and reasoning from concrete to representational approaches.
Design Principle(s): Maximize meta-awareness


aligns: no