Lesson 9Drawing Triangles (Part 1)

Let’s see how many different triangles we can draw with certain measurements.

Learning Targets:

Given two angle measures and one side length, I can draw different triangles with these measurements or show that these measurements determine one unique triangle or no triangle.

9.1 Which One Doesn’t Belong: Triangles

Which one doesn’t belong?

Triangles A, B, C, and D all have different angles and side lengths.

9.2 Does Your Triangle Match Theirs?

Three students have each drawn a triangle. For each description of a student’s triangle:

Drag the vertices to create a triangle with the given measurements.
Compare their measurements to the other side lengths and angle measures in your triangle.
Decide whether the triangle you made must be an identical copy of the triangle that the student drew. Explain your reasoning.

Jada’s triangle has one angle measuring 75°.

Andre’s triangle has one angle measuring 75° and one angle measuring 45°.

Lin’s triangle has one angle measuring 75°, one angle measuring 45°, and one side measuring 5 cm.

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9.3 How Many Can You Draw?

Draw as many different triangles as you can with each of these sets of measurements:
1. Two angles measure , and one side measures 4 cm.
2. Two angles measure , and one side measures 4 cm.
3. One angle measures , one angle measures , and one side measures 4 cm.
Which sets of measurements determine one unique triangle? Explain or show your reasoning.

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Are you ready for more?

In the diagram, 9 toothpicks are used to make three equilateral triangles. Figure out a way to move only 3 of the toothpicks so that the diagram has exactly 5 equilateral triangles.

Lesson 9 Summary

Sometimes, we are given two different angle measures and a side length, and it is impossible to draw a triangle. For example, there is no triangle with side length 2 and angle measures and :

In the figure a horizontal line segment is drawn and labeled 2. On the left end of the line segment, a dashed line is drawn upward and to the left. The angle formed between the dashed line and the horizontal line is labeled 120 degrees. On the right end of the horizontal line, a dashed line is drawn upward and to the right. The angle formed between the dashed line and horizontal line is labeled 100 degrees.

Sometimes, we are given two different angle measures and a side length between them, and we can draw a unique triangle. For example, if we draw a triangle with a side length of 4 between angles and , there is only one way they can meet up and complete to a triangle:

Two intersecting dotted lines form a triangle with a side length of 4 and angles of 60 and 90 degrees.

Any triangle drawn with these three conditions will be identical to the one above, with the same side lengths and same angle measures.

Lesson 9 Practice Problems

Use a protractor to try to draw each triangle. Which of these three triangles is impossible to draw?
1. A triangle where one angle measures and another angle measures
2. A triangle where one angle measures and another angle measures
3. A triangle where one angle measures and another angle measures
A triangle has an angle measuring , an angle measuring , and a side that is 6 units long. The 6-unit side is in between the and angles.
1. Sketch this triangle and label your sketch with the given measures.
2. How many unique triangles can you draw like this?
1. Find a value for that makes less than .
2. Find a value for that makes greater than .
A factory produces 3 bottles of sparkling water for every 7 bottles of plain water. If those are the only two products they produce, what percentage of their production is sparkling water? What percentage is plain?
Lin’s mom bikes at a constant speed of 12 miles per hour. Lin walks at a constant speed of the speed her mom bikes. Sketch a graph of both of these relationships.