The perimeter of a rectangle

Follow the description and watch the results! Try to solve the tasks beneath the applet.
Use the slider to move the point slowly up and down. Look what happens![br]Repeat the procedure with each point.
In the task above you can see a paled property. What do you think is meant by "the perimeter"?
Use your notebook: Draw a regtangle with the length a und the width b. Write down the three formulas for the perimeter you have discovered.
Explain in your own words why we can use the formula [math]U=2\cdot\left(a+b\right)[/math] for calculating the perimeter of a rectangle.
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– Tanja Wassermair, Konrad Maier

Information: The perimeter of a rectangle