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A triangle has three sides, but not just any set of three lengths will make a triangle. Use linkagestrips to answer Problems B1B5. Note 3
Fill in the table below. Try to build triangles with the given lengths. Write “yes” or “no” in the fourth column of the table to indicate whether you can or cannot make a triangle from those three lengths. Experiment with different sets of lengths. When you build a triangle, see if you can deform it (change its shape) into a different triangle while keeping the side lengths the same. Scroll down the page to Solutions to see the filledin table. Note 4
Side B 
Side C 
Is it a triangle? 
Can it be deformed? 

4 units  4 units  4 units  
4 units  3 units  2 units  
3 units  2 units  1 units  
Suppose you were asked to make a triangle with sides 4, 4, and 10 units long. Do you think you could do it? Explain your answer. Keep in mind the goal is not to try to build the triangle, but to predict the outcome.
Come up with a rule that describes when three lengths will make a triangle and when they will not. Write down the rule in your own words.
Suppose you were asked to make a triangle with sides 13.2, 22.333, and 16.5 units long. Do you think you could do it? Explain your answer.
To answer this question, you will need to know what it means for two triangles to be “different.” One definition says that triangles that are “different” cannot have the exact same size and shape. Rotating or reflecting a triangle with the same size and shape does not produce a “different” triangle.
Can a set of three lengths make two different triangles?
For this activity, use linkage strips or make your own. See Note 1.
Fill in the table below. Use this linkagestrip Interactive Activity (or the handson version) to try to build quadrilaterals with the given lengths. Write “yes” or “no” in the fifth column of the table to indicate whether or not you can make a quadrilateral from those four lengths. Experiment with different sets of lengths. When you build a quadrilateral, see if you can deform it into a different quadrilateral with the same side lengths. When you click “Show Answers,” the filledin table will appear below the problem. Scroll down the page to see it.
For some of the lengths above, can you connect them in a different order to make a different quadrilateral? If so, which ones? How is this different from building triangles?
Come up with a rule that describes when four lengths will make a quadrilateral and when they will not. Write down the rule in your own words. (You may want to try some more cases to test your rule.)
Can a set of four lengths make two different quadrilaterals?
Linkagestrip problems adapted from IMPACT Mathematics Course, 1, developed by Education Development Center, Inc. pp. 5556, © 2000 Glencoe/McGrawHill. Used with permission. www.glencoe.com/sec/math
The triangle inequality is a famous result in mathematics. It says that for three lengths to make a triangle, the sum of any two sides must be greater than the third side. Often you will see a picture like this, where a, b, and c represent the three lengths of the sides.
The triangle inequality is the mathematical statement of the old adage, “The shortest distance between two points is a straight line.” If you don’t travel along the straight line, you travel two sides of a triangle, and that trip takes longer.
You have also probably found that triangles are rigid. That is, if a set of lengths makes a triangle, only one triangle is possible. You can’t push on the vertices to make a different triangle with the same three sides. Triangles are the only rigid polygon, which makes them quite useful for construction.
This property is often abbreviated as SSS (sidesideside) congruence. If the three sides of one triangle have the same lengths as the three sides of another triangle, then the two triangles are congruent. That is, they have exactly the same size and shape. All of the angle measurements will match, as will other measurements, such as their areas, the lengths of the corresponding altitudes, and so on. If you cut the two triangles out from a piece of paper, you could fit one exactly on top of the other.
Use linkage strips (or make your own strips) to answer Problems B1B5.
If you are working in groups to construct triangles and then, in Problem B6, quadrilaterals, discuss the rules you came up with for instances in which three lengths make a triangle and in which four lengths make a quadrilateral.
Sometimes people may be confused about the question of whether or not you can “deform” one triangle into anther one. It helps to build another figure — use five sides so as not to give away the answers for the quadrilaterals in Problem B6 — and see how easy it is to push on the sides and angles to form many different pentagons with the same five sides. Then, although the materials may allow a little bit of “wiggle,” it becomes clear whether or not you can deform a triangle into a completely different one.
Here is the table filled in for the triangles with given lengths and for other sample triangles.
Side A 
Side B 
Side C 
Is it a triangle? 
Can it be deformed? 
4  4  4 
Yes 
No 
4  3  2 
Yes 
No 
3  2  1 
No 
N/A 
4 
3 
2 
Yes 
No 
1 
2 
4 
No 
N/A 
2 
4 
4 
Yes 
No 
3 
1 
1 
No 
N/A 
2 
3 
3 
Yes 
No 
2 
4 
2 
No 
N/A 
Other answers will vary individually, but no triangle will be deformable.
No, this is not possible. If we attach the two sides of lengths of 4 units to the endpoints of the side of length 10, the first two sides will not meet at a point to create a triangle. Together they are too short.
Three lengths can form a triangle only if the sum of the lengths of any two sides is greater than the length of the third side.
Yes. Because the sum of lengths of any two sides is greater than the length of the remaining side, the two sides will be able to meet at a point and create a triangle when attached to the endpoints of the third side.
No, three fixed lengths determine one and only one triangle. This is demonstrated by the fact that none of the triangles found in Problem B1 can be “deformed” into a different shape.
Here is the table filled in for the quadrilaterals with given lengths and other sample quadrilaterals.
Side A 
Side B 
Side C 
Side D 
Is it a quadri 
Can it be deformed? 
4  4  4  4 
Yes 
Yes 
4  3  2  2 
Yes 
Yes 
3  2  1  1 
Yes 
Yes 
4  1  2  1 
No 
N/A 
1 
1 
1 
4 
No 
N/A 
2 
2 
2 
2 
Yes 
Yes 
1 
4 
3 
1 
Yes 
Yes 
1 
3 
3 
4 
Yes 
Yes 
2 
3 
4 
1 
Yes 
Yes 
4 
1 
1 
2 
No 
N/A 
Other answers will vary individually, but all quadrilaterals will be deformable.
As long as no more than two sides of a quadrilateral are equal in length, we can reorder the way the sides are connected and obtain a different quadrilateral. This is not the case with triangles: If we reorder the sides, we get the same triangle.
Four lengths can form a quadrilateral as long as the sum of the lengths of any three sides is greater than the length of the fourth side.
Yes. For example:
Also, if the sides are not the same length, ordering them differently will produce different quadrilaterals.