Constructing Triangles

Common Core 7.G.2

Vocabulary• Uniquely defined• Ambiguously defined• Nonexistent

Triangle Inequality Theorem

The sum of the lengths of any two sides of a triangle is greater than the length of the third side.

x z

y

PracticeCan these measures be the sides of a triangle?

1. 7, 5, 42. 2, 1, 53. 9, 6, 34. 7, 8, 4

PracticeCan these measures be the sides of a triangle?

1. 7, 5, 4 yes2. 2, 1, 5 no3. 9, 6, 3 no4. 7, 8, 4 yes

Example 1Using the measurements 6 in and 8 in, what is the smallest possible length of the third side? What is the largest possible length of the third side?

Example 1If you assume that 6 and 8 are the shorter sides, then their sum is greater than the third side. Therefore, the third side has to be less than 14.

Example 1If you assume that the larger of these values, 8, is the largest side of the triangle, then 6 plus the missing value must be greater than 8. Therefore, the third side has to be more than 2.

Example 1If you put these two inequalities together, then you get the range of values that can be the length of the third side:

Therefore, any value between 2 and 14 (but not equal to 2 or 14) can be the length of the third side.

PracticeSolve for the range of values that could be the length of the third side for triangles with these 2 sides:

1. 2 and 62. 9 and 113. 10 and 18

(Be sure to look for patterns!)

PracticeSolve for the range of values that could be the length of the third side for triangles with these 2 sides:

1. 2 and 6 2. 9 and 113. 10 and 18

What patterns do you see?

How many triangles can be constructed?

Remember our “I can” statement:“I can determine if 1, more than 1,

or no triangles can be constructed given 3 side or 3 angle measures.”

• The organizer below should be filled out and glued in your notebook.

Triangles are:Nonexistent - If three side lengths or angle measures do not make a triangle, you would say that the triangle is nonexistent because a triangle cannot be formed.

Unique - If three side lengths do make a triangle, you would say that the triangle is unique because it creates one, specific triangle.

Ambiguous – If three angle measures do make a triangle, you would say that the triangle is ambiguous because it creates more than 1 triangle.

The Triangle is…  

Ambiguous Unique Non-ExistentDefinition: Able to draw more than 1 triangle

Definition: Only able to draw 1 triangle

Definition: Not possible to draw a triangle

Use when given 3 angles that equal 180˚.

Use when given 3 side lengths the satisfy the Triangle Inequality Theorem.

Use when the two theorems WILL NOT work.

Ambiguous Triangles• Used when 3 angle measures add up to equal

180˚.

• Look at these triangles. They have the same angle measurements, which is why they are similar in shape. However, do they have the same side lengths? No.

• This proves why more than 1 triangle can be drawn.

Unique Triangles• Used when 3 side lengths are given and satisfy

the Triangle Inequality Theorem.

• Look at the triangle below. It has 3 side lengths that will make a triangle. You can flip it, rotate it, or translate it, but there is still ONLY ONE triangle that can be made.

Non-existent Triangles• When angle measures or side lengths DO NOT

satisfy our 2 theorems, no triangles can be created.

• If the sum of the 2 smaller sides is NOT greater than the longest side, it WILL NOT make a triangle.

• If the sum of the angle measures DO NOT equal 180˚, it WILL NOT make a triangle.

Angles of Triangles

What do they create? A straight line, which is equal to 180 degrees; therefore, the sum of the angles in a triangle always equal 180 degrees. This is called the Triangle Angle Sum Theorem. Glue your triangle corners in your math notebook and explain this in your own words.

PracticeGiven the following angle measurements, determine the third angle measurement.

Do these measurements create triangles?3. 4.

PracticeGiven the following angle measurements, determine the third angle measurement. 1.2.

Do these measurements create triangles?3. yes4. no

Constructing Triangles from Angles

Look at these triangles. They have the same angle measurements, which is why they are similar in shape. However, do they have the same side lengths?

Constructing Triangles from Angles

Look at these triangles. They have the same angle measurements, which is why they are similar in shape. However, do they have the same side lengths? No.

Since they aren’t the same size, will angle measurements construct unique triangles?

Constructing Triangles from Angles

Look at these triangles. They have the same angle measurements, which is why they are similar in shape. However, do they have the same side lengths? No.

Since they aren’t the same size, will angle measurements construct unique triangles? No.

Constructing Triangles from Angles

Conditions, such as angle measurements, that can create more than one triangle are called ambiguously defined.

SummaryTake turns with your partner explaining the Triangle Angle Sum Theorem and the Triangle Inequality Theorem in your own words.