Day 5 Activities


Areas of Regular Polygons and Circles


A regular polygon has congruent sides and congruent angles. You can divide any polygon into congruent triangles by drawing segments from the center to each vertex. You can use the area of one of these triangles to find the area of the polygon. You can extend this method to derive a formula for the area of a circle. In this activity, you'll manipulate a sketch in which you can vary the number of sides of a regular polygon to see how that affects the area.

Sketch and Investigate

1. Click here to open the sketch Circle

2. Drag (or animate) point "drag" along its segment. Observe how the polygon and the measures change.

3. Drag to give the polygon four or more sides.

Question 1. Each regular polygon with four or more sides is divided into triangles. look at the triangle with a thick outline. It has segments labeled a for apothem and s for side. The side is the base of this triangle and the length of the apothem is the height of the triangle. Write a formula for the area of the triangle using a and s.


 

 

When farmers Clarence and Mytle died, they left their daughters their land with instructions to divide it equally. One daughter, Ella, was considerably more conniving than her sister, Jo. The land, unfortunately, was shaped as an irregular quadrilateral, and it wasn't immediately obvious how to divide it equally. Ella first tried to get Jo to agree to split it down the diagonal AC as shown, with Ella getting region ACD and Jo getting region ABC. Jo could see that was a bad deal.

 

 

 

 

Ella then offered to split the land with both diagonals. Ella would take two regions, AED and BEC, leaving Jo with regions ABE and CED. This sounded good to Jo, but when she checked it out, she found that the sums of the regions were still not equal. "Ah," said Ella, "but the product of our regions are equal!" This stumped Jo, and she agreed to the deal out of sheer awe for Ella's discovery.

 

 

 

Sketch and Investigate

Model this problem with Sketchpad. Is Ella's claim true for all quadrilaterals? Does that mean that this was a fair way to divide the land? Why or why Not? See if you can show why Ella's conjecture is true. Record your findings.

Click Here for GSP sketch.

Explore More

Come up with a way to divide an irregular quadrilateral using only two segments so that the four regions can be divided equally between two people.

 

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