Lecture 4, MATH 210G.02, Fall 2016 Greek Mathematics and Philosophy Goals: Learn a few of the theorems proved by Greek mathematicians Understand some of the Greek philosophy in which mathematics was developed

Lecture 4, MATH 210G.02, Fall 2015 Greek Mathematics and Philosophy Period 1: 650 BC-400 BC (pre-Plato) Period 2: 400 BC – 300 BC (Plato, Euclid) Period 3: 300 BC – 200 BC (Archimedes, Appolonius, Eratosthenes

 Thales ( BC): father of mathematical proof

In the diagram, the ratio of the segments AD and DB is the same as the ratio of the segments AE and EC A) True B) False

Pythagoras (c BC)

In the windmill diagram, the area of the square with side a plus the area of the square with side b equals the area of the square with side c A)True B)False

Pythagorean philosophy ☺ Transmigration of souls, ☺ purification rites; developed rules of living believed would enable their soul to achieve a higher rank among the gods. ☺ Theory that numbers constitute the true nature of things, including music

The diatonic: ratio of highest to lowest pitch is 2:1, produces the interval of an octave. Octave in turn divided into fifth and fourth, with ratios 3:2 and 4:3 … up a fifth + up a fourth = up an octave. fifth … divided into three whole tones, each corresponding to the ratio of 9:8 and a remainder with a ratio of 256:243 fourth into two whole tones with same remainder. harmony… combination… of … ratios of numbers … whole cosmos … and individual do not arise by a chance combinations … must be fitted together in a "pleasing" (harmonic) way in accordance with number for an order to arise. CDEFGABC 19/881/644/33/227/16243/1282

πBelieved the number system … and universe… based on their sum (10) π… swore by the “Tetractys” rather than by the gods. πOdd numbers were masculine and even were feminine. πHippasos …discovered irrational numbers…was executed. πHints of “heliocentric theory” πdiscovery that music was based on proportional intervals of numbers 1—4

"Bless us, divine number, thou who generated gods and men! O holy, holy Tetractys, thou that containest the root and source of the eternally flowing creation! For the divine number begins with the profound, pure unity until it comes to the holy four; then it begets the mother of all, the all-comprising, all- bounding, the first-born, the never-swerving, the never-tiring holy ten, the keyholder of all"

Clicker question The number 10 is a perfect number, that is, it is equal to the sum of all of the smaller whole numbers that divide into it. A) True B) False

…it was better to learn none of the truth about mathematics, God, and the universe at all than to learn a little without learning all Pythagoreans … believed… when someone was "in doubt as to what he should say, he should always remain silent” Pythagoreans’ inner circle,“mathematikoi” ("mathematicians”); outer circle, “akousmatikoi” ("listeners”) … the akousmatikoi were the exoteric disciples who… listened to lectures that Pythagoras gave out loud from behind a veil. Pythagorean theory of numbers still debated among scholars. Pythagoras believed in "harmony of the spheres”… that the planets and stars moved according to mathematical equations, which corresponded to musical notes and thus produced a symphony

Music of the Spheres

The square root of two is a rational number (the ratio of two whole numbers) A)True B)False

The Pythagorean Theorem

Which of the two diagrams provide “visual proof” of the Pythagoran theorem? A)Left diagram only B) Right diagram only C) Both diagrams together

Plato (428 BC – 348 BC),

Plato’s Cave Analogy

Note ratios: AB:BC :: CD:DE:: AC:CE

In Plato’s Divided Line, Mathematics falls under the following category: A)Highest form of true knowledge B)Second highest form of true knowledge C)A form of belief, but not true knowledge D)A form of perception

Plato (left) and Aristotle (right)

Aristotle (384 BC – 322 BC) Aristotle’s logic: the syllogism Major premise: All humans are mortal. Minor premise: Socrates is a human. Conclusion: Socrates is mortal.

EpictetusEpictetus and The Stoics (c 300 BC)The Stoics Stoics believed … knowledge attained through use of reason… Truth distinguishable from fallacy; *even if, in practice, only an approximation can be made. Modality (potentiality vs actuality). Conditional statements. (if…then) Meaning and truth

Euclid’s “Elements” arranged in order many of Eudoxus's theorems, perfected many of Theaetetus's, and brought to irrefutable demonstration theorems only loosely proved by his predecessors Ptolemy once asked him if there were a shorter way to study geometry than the Elements, … In his aim he was a Platonist, being in sympathy with this philosophy, whence he made the end of the whole "Elements" the construction of the so- called Platonic figures.

The axiomatic method The Elements begins with definitions and five postulates. There are also axioms which Euclid calls 'common notions'. These are not specific geometrical properties but rather general assumptions which allow mathematics to proceed as a deductive science. For example: “Things which are equal to the same thing are equal to each other.” ”

Euclid's Postulates A straight line segment can be drawn joining any two points. 2. Any straight line segment can be extended indefinitely in a straight line. 3. Given any straight line segment, a circle can be drawn having the segment as radius and one endpoint as center. 4. All right angles are congruent. 5. If two lines are drawn which intersect a third in such a way that the sum of the inner angles on one side is less than two right angles, then the two lines inevitably must intersect each other on that side if extended far enough. This postulate is equivalent to what is known as the parallel postulate. Euclid's fifth postulate cannot be proven from others, though attempted by many people. Euclid used only 1—4 for the first 28 propositions of the Elements, but was forced to invoke the parallel postulate on the 29th. In 1823,Bolyai and Lobachevsky independently realized that entirely self-consistent "non- Euclidean geometries" could be created in which the parallel postulate did not hold.

Euclid's Postulates 1. A straight line segment can be drawn joining any two points. 2. Any straight line segment can be extended indefinitely in a straight line. 3. Given any straight line segment, a circle can be drawn having the segment as radius and one endpoint as center. 4. All right angles are congruent. 5. If two lines are drawn which intersect a third in such a way that the sum of the inner angles on one side is less than two right angles, then the two lines inevitably must intersect each other on that side if extended far enough. This postulate is equivalent to what is known as the parallel postulate.

Euclid's fifth postulate cannot be proven from others, though attempted by many people. Euclid used only 1—4 for the first 28 propositions of the Elements, but was forced to invoke the parallel postulate on the 29th. In 1823,Bolyai and Lobachevsky independently realized that entirely self-consistent "non-Euclidean geometries" could be created in which the parallel postulate did not hold.

Non-Euclidean geometriesNon-Euclidean geometries2 Non-Euclidean geometriesNon-Euclidean geometries 3 Non-Euclidean geometriesNon-Euclidean geometries 1

Clicker question Euclid’s fifth postulate, the “parallel postulate” can be proven to be a consequence of the other four postulates A) True B) False

Archimedes Possibly the greatest mathematician ever; Theoretical and practical

Other cultures Avicenna ( ): propositional logic ~ risk analysis Parallels in India, China, Medieval ( ) Occam ( )

Exercises

Explain how the Pythagorean theorem follows from the picture using the formula for the area of a trapezoid

Explain how the Pythagorean theorem follows from the picture

Prove that the area of the big hexagon is the sum of the areas of the smaller ones

Assuming: the area of a semicircle of diameter d is Prove that the area of the big semicircle is the sum of the areas of the smaller ones

Some practice problems If a=3 and b=4, what is the length c of the hypotenuse of the triangle? 3 4 c

If a=5, b=4, c=3, d=3, and e=√5, find f. 3 4 f a c b d e

Explain the lengths of the sides of the Pythagorean spiral

A ladder is 10 feet long. When the top of the ladder just touches the top a wall, the bottom of the ladder is 6 feet from the wall. How high is the wall?

TV screen size is measured diagonally across the screen. A widescreen TV has an aspect ratio of 16:9, meaning the ratio of its width to its height is 16/9. Suppose that a TV has a one inch boundary one each side of the screen. If Joe has a cabinet that is 34 inches wide, what is the largest size wide screen TV that he can fit in the cabinet?

Advanced The spherical law of cosines states that, on a spherical triangle. Cos (c/R) = (cos a/R) (cos b/R) + (sin a/R) (sin b/R) cos γ where R is the radius of the sphere. If the Earth’s radius is 6,371 km, find the distance from: from Seattle (48°N, 2°E) to Paris (48°N, 122°W) if traveling due east? from Lincoln, NE (40°N, 96°W) to Sydney, Australia (34°S, 151°E).