1. 1 The number  is defined as the ratio of the circumference of a circle to its diameter. What is  ? CS110: Introduction to Computer Science: Lab Module 3 How can you compute  ? Archimedes and Pi Prepared for SSAC by Eric Gaze Alfred University, Alfred, NY © The Washington Center for Improving the Quality of Undergraduate Education. All rights reserved. 2005 Quantitative concepts and skills Estimation Ratios Limits Proportions Pythagorean Theorem Geometrical Reasoning Logic Functions, IF

2. 2 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. 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.

3. 3 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. To find  we need to find the circumference… Can you subdivide this hexagon into a collection of right triangles?

4. 4 Which ratio, or, is more useful for computing ? How? To approximate  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. b a c Note that Multiplying the numerator by the number of sides will give:

5. 5 Use the Pythagorean Theorem to compute the height of this triangle in terms of x. Archimedes started with a circumscribed hexagon, giving a right triangle well known to the Greeks. This triangle is a 30-60-90 right triangle, which is ½ of an equilateral 60-60-60 triangle. Thus the ratio of sides in a 30-60- 90 right triangle: b a c What is special about this triangle? x ½ x

6. 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. Recreate this spreadsheet with appropriate formulas. = Given Number = Formula 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”) 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.

7. 7 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. Now we iterate

8. 8 Archimedes is now ready to estimate  using a hexagon. 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 30-60-90 right triangles. Recreate this spreadsheet. Refer back to slide 4 for the approximation formula. Use your work from slides 5 and 6 to estimate . Better Approximation!

9. 9 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. Archimedes needs to find the new ratio: b2b2 b1b1 b a c c2c2 How many sides does the new polygon have? Proposition 3 from Book VI of Euclid’s Elements: (for the case where the angle opposite b 1 is the same as the angle opposite b 2 ) For more information on this equation, click here The approach...

10. 10 Start with Proposition VI from Book 3 of Euclid’s Elements: Archimedes used Euclid’s Proposition VI to arrive at the desired ratio as follows: b2b2 b1b1 b a c c2c2 What do we need to multiply both sides by to get the ratio ? in the sky First, Add 1 to Both Sides! To get to

11. 11 Continuing the calculation from the previous slide we arrive at as follows: b2b2 b1b1 b a c c2c2 Proposition 3 from Book VI of Euclid’s Elements: How is related to and ? b2b2

12. 12 We are now ready to estimate  using a dodecagon. The ratio c/b is not necessary for estimating . If we knew the ratio, c 2 /b 2 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. Recreate this spreadsheet b1b1 b a c c2c2 b2b2

13. 13 Use the Pythagorean Theorem for the smaller green triangle and divide through by b 2. Solve for c 2 /b 2. 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  using polygons with as many sides as we please! Use the formula you found for c 2 /b 2 to recreate this spreadsheet. b1b1 b a c c2c2 b2b2

14. 14 Expand your spreadsheet from the previous slide. Hi

15. 15 1.Write out your calculations from slide 5 involving the Pythagorean Theorem. Verify the ratios for a/b and c/b. 2.Write out your answer to the question from Slide 8 with clear explanation and a picture. 4.Write out your answers to the questions on Slides 9 and 10 with clear explanations. 5.Write out your calculations from Slide 12 involving the Pythagorean Theorem. 6.Submit an electronic copy of your spreadsheet from Slide 13 on Blackboard. 7.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 . 9.Submit an electronic copy of your spreadsheet including your answers on Blackboard Digital Dropbox. Goodbye End of Module Assignment

16. 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 http://aleph0.clarku.edu/~djoyce/java/elements/bookVI/propVI3.html, which also contains further information about the proposition, including a proof. http://aleph0.clarku.edu/~djoyce/java/elements/bookVI/propVI3.html Back to Slide 9