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6.5 Understanding the Pythagorean Relationship Using Interactive Figures

This activity presents one dynamic version of a demonstration of the Pythagorean relationship. Visual and dynamic demonstrations can help students analyze and explain mathematical relationships, as described in the Geometry Standard.

The interactive figure in this activity can help students understand the Pythagorean relationship and gives them experience with transformations that preserve area but not shape.

The following links to an updated version of the e-example.

6-5-50x50.jpg  Understanding the Pythagorean Relationship (6.5)

This e-example provides a proof without words for the Pythagorean Theorem. 

The e-example below contains the original applet which conforms more closely to the pointers in the book.

Icon_6-5sm.gif  The Pythagorean relationship, a2 + b2 = c2 (where a and b are the lengths of the legs of a right triangle and c is the hypotenuse), can be demonstrated in many ways, including with visual "proofs" that require little or no symbolism or explanation.


In this task, you will explore a dynamic demonstration of the Pythagorean relationship. Your goal is to determine how the interactive figure demonstrates the Pythagorean relationship. Consider the blue right triangle in the interactive figure below. Red and yellow squares have been constructed on its legs. A square has also been constructed on its hypotenuse. Click on the yellow square; notice how the outline of a parallelogram appears. Drag the yellow square to transform it into the parallelogram. Now drag the parallelogram to transform it into the rectangle that appears within the large square. Repeat this process for the red square. How does your transformation of the squares into parallelograms and then into rectangles affect their area? What relationship is demonstrated when the rectangles fill the large square formed on the hypotenuse? Now drag the red and yellow rectangles back to their original positions as squares on the legs of the triangle. Change the blue right triangle by dragging it by a vertex and repeat the transformation of the yellow and red squares. What do you observe?



How to Use the Interactive Figure 

The red and yellow squares on the legs of the triangle can be transformed into parallelograms and then into rectangles inside the square attached to the hypotenuse.

To move the yellow region, click and drag it toward the vertex opposite the hypotenuse. Once in this position, the yellow region is a parallelogram. Click and drag the region through the triangle and into the square attached to the hypotenuse. The same process will work on the red square. Reversing this process can be used to move the regions back to their original positions.

To modify the right triangle, click and drag on any vertex. Clicking and dragging on a side of the triangle will change the triangle's position.

The Show or Hide Lines button displays or hides a set of guide lines for the activity. 


Suppose a and b represent the lengths of the legs of the blue right triangle, and c represents the length of its hypotenuse. By engaging with the interactive figure, students observe that the squares formed on the two legs of the triangle (with areas a2 and b2) can be transformed to completely fill the square formed on the hypotenuse (with area c2). That is, the sum of the areas of the squares formed on the two legs is equal to the area of the square formed on the hypotenuse, or a2 + b2 = c2. Teachers can ask students to make a general observation about what happens as the areas of the squares on the legs of a right triangle are transformed into parallelograms that will finally fit into the area of the square on the hypotenuse. Students could also be asked to justify their observations by attending to areas that remain constant throughout the process of transforming a shape (a shear transformation applied to parallelograms). Finally, the original right triangle can be transformed in various ways, and the process of fitting the squares formed on the legs into the square on the hypotenuse repeated. Teachers should encourage their students to consider why it is important to repeat the demonstrations for different right triangles. 

Interesting extensions to this task include the following: (a) Consider what relationships exist among the areas of similar figures, other than squares, built on the legs and hypotenuse of any right triangle; (b) consider what relationships exist among the areas of the squares built on the three sides of an obtuse or acute triangle rather than a right triangle. 

Take Time to Reflect 

  • How is doing this activity on the computer different from working with physical models such as dot paper or geoboards?
  • How can students use the approach in this activity to construct a proof of the Pythagorean relationship?


Based on an idea provided by Colette Laborde, EIAH, Laboratorie Leibniz-IMAG, at the 1999 ENC-NCTM Conference and Workshop: "The Role of Technology and Examples in the Principles and Standards Document."


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