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Geometry Science Fair Project

Polygon Sides and Pi Estimation Error

Hard
Polygon Sides and Pi Estimation Error | Science Fair Projects | STEM Projects
How fast does your estimate of pi improve when you use polygons with more sides? Ancient math used shapes drawn inside and around circles to estimate pi. The more sides the polygon has, the closer the estimate gets. You calculate pi by averaging the perimeters of polygons inscribed in and drawn around a circle. Then you compare the errors from polygons with different numbers of sides. You test six different pairs and track how the error ratio changes as the side count grows. The error ratio follows a clear pattern. It approaches the square of the inverse of the side-count ratio. This means the error shrinks in a predictable way as you add more sides.

Hypothesis

The hypothesis is that the ratio between the error in determining pi using by inscribing polygons within and circumscribing polygons about a circle with (km) sides and that obtained using polygons with (kn) sides will approach (n/m)^2 as k increases.

Science Concepts Learned

Pi

Because pi equals a circle's edge length divided by its width, any polygon drawn inside or around a circle gives an approximate value. Ancient math used exactly this approach — inscribed and circumscribed shapes whose perimeters bracket the true value. The more sides the polygon has, the closer the estimate gets. You can track how fast the improvement happens by comparing errors from polygons with different side counts across six different pairs. The error ratio follows a clear pattern: as the side count grows, it approaches the square of the inverse of the side-count ratio, meaning the error shrinks in a predictable, measurable way.

Polygons

Shapes drawn inside and around a circle act as lower and upper bounds for pi — one flat outline sits just within the curve, the other just outside it. As you test six different pairs and track how the error ratio changes, a clear pattern emerges: the gap between the two estimates shrinks in a predictable way. It approaches the square of the inverse of the side-count ratio, meaning each doubling of sides yields a reliably smaller error.

Circumference

Every circle's circumference connects to pi through the fixed ratio C = πd, and ancient mathematicians narrowed in on that value by drawing polygons inscribed in and around circles. The more sides a polygon has, the closer its perimeter gets to the true circumference. You calculate pi by averaging the perimeters of polygons drawn inside and around a circle, and the error ratio follows a clear pattern — shrinking in a predictable way as you add more sides.

Geometric Convergence

Ancient mathematicians estimated pi by drawing polygons inside and around circles — and the more sides a polygon has, the closer the estimate gets. You calculate pi by averaging the perimeters of polygons inscribed in and circumscribed about a circle, then track how the error changes as the side count grows. Testing six different pairs of polygons, a clear pattern emerges: the error ratio approaches the square of the inverse of the side-count ratio. That means the error shrinks in a predictable way as you add more sides, giving you a precise measure of how fast the shape converges toward a perfect circle.

Inscribed and Circumscribed Figures

Ancient math used shapes drawn inside and around circles to estimate pi. When you inscribe a polygon within a circle and circumscribe another around it, you get estimates from both sides. You calculate pi by averaging their perimeters, then track how the error changes as the side count grows. Testing six different pairs reveals a clear pattern: the error ratio approaches the square of the inverse of the side-count ratio. That means as you add more sides, the error shrinks in a predictable, measurable way.

Perimeter

Ancient mathematicians estimated pi by drawing polygons inside and around circles, then measuring the total distance around each shape. You do the same: average the perimeters of polygons inscribed in and circumscribed about a circle to get your pi estimate. As you increase the number of sides, each perimeter traces the circle's edge more closely, and the estimate improves. When you test six different polygon pairs and track how the error ratio changes, a clear pattern emerges — the error shrinks in a predictable way as the side count grows.

Error Analysis

How far does your estimate of pi land from the true value? In this experiment, you use polygon perimeters to estimate pi, then calculate the gap between your result and actual pi. As the polygon gains more sides, that gap shrinks — and it shrinks in a predictable pattern. The error ratio approaches the square of the inverse of the side-count ratio, meaning each increase in sides compresses the error by a calculable factor.

Method & Materials

You will develop formulas to determine the perimeters of the regular polygons inscribed within and circumscribed about a circle. You will estimate pi by using the expression: (X(sin(180/X) + X(tan(180/X))) / 2, and calculate the error in estimating pi using polygons with the formula: error = ((X(sin(180/X) + X(tan(180/X))) / 2) - pi.
You will need formulas to determine the perimeters of the regular polygons inscribed within and circumscribed about a circle, and a calculator.

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Results

The results of the experiment showed that the graphs were consistent with the hypothesis. As the number of sides increased, the error ratio approached the square of the inverse of the ratio of the number of sides.

Why do this project?

This science project is interesting because it explores the ratio between the error in estimating pi using polygons with different numbers of sides.

Also Consider

Experiment variations to consider include changing the number of sides of the polygons, or changing the diameter of the circle.

Full project details

Additional information and source material for this project are available below.
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