1. Archimedes and Pi What is p? How can you compute p? Module Arch-Pi 1
The number p is defined as the ratio of the circumference of a circle to its diameter.
Quantitative concepts and skills
Estimation
Ratios
Limits
Proportions
Pythagorean Theorem
Geometrical Reasoning
Logic Functions, IF
Prepared for SSAC by
Eric Gaze
Alfred University, Alfred, NY
© The Washington Center for Improving the Quality of Undergraduate Education. All rights reserved. 2005

2. Overview of Module
Archimedes of Syracuse, 287 – 212 BCE, was a brilliant Greek mathematician credited with the first sophisticated approximation of p. Ancient civilizations knew this ratio was slightly larger than 3, but the Greeks were the first to explore exactly how much larger it is.
Slides 3-4 introduce the approach Archimedes took to approximate pi by using circumscribed polygons.
Slides 5-7 give a first approximation using a circumscribed hexagon and explore the related problem of estimating square roots.
Slides 8-12 explore Archimedes’ iterative process of cascading right triangles that share the same vertex angle, which is bisected at every step.
Slide 13 creates the spreadsheet which gives the approximation of pi using circumscribed polygons.
Slide 14 gives the assignment to hand in.

3. To find p we need to find the circumference…
How can you measure the arc-length of a circle?
The ingenious technique devised by the Greeks and adroitly exploited by Archimedes was to circumscribe a polygon about the circle. The perimeter of the polygon then approximates the circumference!
Polygons are made up of right triangles and straight sides, perfect for a civilization that included Euclid and Pythagoras.
Can you subdivide this hexagon into a collection of right triangles?

4. Which ratio, or , is more useful
To approximate p we need to compute the ratio of the perimeter to the diameter…
Archimedes focused on a single right triangle and the associated ratio of the two legs of the right triangle. c b a
Which ratio, or , is more useful
for computing ? How?
Note that
Multiplying the numerator by the number of sides will give:

5. What is special about this triangle?
Archimedes started with a circumscribed hexagon, giving a right triangle well known to the Greeks.
What is special about this triangle?
This triangle is a right triangle, which is ½ of an equilateral triangle. x b
½ x c a
Use the Pythagorean Theorem to compute the height of this triangle in terms of x.
Thus the ratio of sides in a right triangle:

6. Archimedes now has a big problem: how to compute ?
We can create a spreadsheet to estimate We know this number is between 1 and 2 since
= Given Number
Recreate this spreadsheet with appropriate formulas.
= Formula
To begin the spreadsheet, we must choose a starting value and an increment to increase it by. For our first iteration, we will use a starting value of 1 and an increment of 0.1.
Cells in the first column simply add the increment stored at the top to the number in the row above the cell. Cells in the second column have two possible outputs: the square of “value” or “TOO BIG”. We use the IF logic function:
=IF (B6^2 < 3, B6^2, “TOO BIG”)

7. Add four more columns to your spreadsheet to find
Now we iterate
Use the copy and paste commands to transfer your IF function to each successive, two-column block.
Add four more columns to your spreadsheet to find
to 5 decimal places.
Choose your starting value based on the best estimate from the previous iteration.
We get a very good estimate of the square root of 3 by using five decimal places in the iteration.

8. Archimedes is now ready to estimate p using a hexagon.
Recreate this spreadsheet. Refer back to slide 4 for the approximation formula. Use your work from slides 5 and 6 to estimate p.
Archimedes recognized that this estimate was not very close, and he also knew that to get a better approximation he would need to increase the number of sides. His genius was figuring out how to compute the crucial ratio for polygons which have more sides and are not composed of right triangles.
Better Approximation!

9. How many sides does the new polygon have?
The approach...
Archimedes’ approach is to bisect the vertex angle of the right triangle at the center of the polygon thereby creating a new right triangle. This new right triangle belongs to a new polygon with more sides.
How many sides does the new polygon have? c b1 b c2 b2 a
Proposition 3 from Book VI of Euclid’s Elements:
(for the case where the angle opposite b1 is the same as the angle opposite b2)
For more information on this equation, click here
Archimedes needs to
find the new ratio:

10. What do we need to multiply both sides by to get the ratio
Archimedes used Euclid’s Proposition VI to arrive at the desired ratio as follows:
To get to
Start with
Proposition VI from Book 3 of Euclid’s Elements: c b1 b c2 b2 a
What do we need to multiply both sides by to get the ratio ?
in the sky
First, Add 1 to Both Sides!

11. Proposition 3 from Book VI of Euclid’s Elements:
Continuing the calculation from the previous slide we arrive at as follows:
Proposition 3 from Book VI of Euclid’s Elements: b1 b a c c2 b2 b2
How is related to and ?

12. Recreate this spreadsheet
We are now ready to estimate p using a dodecagon.
Recreate this spreadsheet b1 b a c c2 b2
The ratio c/b is not necessary for estimating p. If we knew the ratio, c2/b2 for the smaller triangle, however, we could bisect the angle again and repeat the process for a 24-sided polygon, and then again and again for polygons with more and more sides! Excel excels at this type of iterative process.

13. Use the formula you found for c2/b2 to recreate this spreadsheet.
Use the Pythagorean Theorem for the smaller green triangle and divide through by b2. Solve for c2/b2.
Use the formula you found for c2/b2 to recreate this spreadsheet. b1 b a c c2 b2
We now have a huge advantage over Archimedes, who had to do each calculation one at a time by hand. We can fill the formulas down the columns and estimate p using polygons with as many sides as we please!

14. Expand your spreadsheet from the previous slide.Hi

15. Goodbye End of Module Assignment
Write out your calculations from slide 5 involving the Pythagorean Theorem. Verify the ratios for a/b and c/b.
Submit an electronic copy of your spreadsheet from Slide 6 on Blackboard.
Write out your answer to the question from Slide 8 with clear explanation and a picture.
Write out your answers to the questions on Slides 9 and 10 with clear explanations.
Write out your calculations from Slide 12 involving the Pythagorean Theorem.
Submit an electronic copy of your spreadsheet from Slide 13 on Blackboard.
Use your spreadsheet to determine how many decimal places of accuracy Archimedes was able to get for a 96-sided polygon using to approximate the square root of 3. We do not know how Archimedes arrived at this fraction.
8. Determine how many decimal places of accuracy you need for the square root of 3 to get 9 decimal places of accuracy for p.

16. Euclid’s Elements: Proposition 3, Book VI
What it says:
If an angle of a triangle is bisected by a straight line cutting the base, then the segments of the base have the same ratio as the remaining sides of the triangle; and, if segments of the base have the same ratio as the remaining sides of the triangle, then the straight line joining the vertex to the point of section bisects the angle of the triangle.
This statement comes from which also contains further information about the proposition, including a proof.
Back to Slide 9