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Plane Geometry - Bruce E. Shapiro

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192 SECTION 38. THE PYTHAGOREAN THEOREMwhile a later sutra by Katyayana (c 200 BCE) is more general 3A rope stretched along the length of the diagonal produces anarea which the vertical and horizontal sides make together.It has inundated common culture as well. Gilbert and Sullivan gave theMajor-General “many cheerful facts about the square of the hypotenuse”while the scarecrow in the Wizard of Oz mistakenly quotes it as:The sum of the square roots of any two sides of an isoscelestriangle is equal to the square root of the remaining side. Oh,joy! Oh, rapture! I’ve got a brain! 4Even Homer Simpson has had a word to say about it, and at least onepresident of the United States (James Garfield) is credited with finding anew proof. 5Theorem 38.1 (Pythagorean Theorem) Let △ABC be a right trianglewith right angle at vertex C. Then if a, b, and c are the sides oppositevertices A,B, and C, respectively,a 2 + b 2 = c 2Figure 38.1: Notation for the Pythagorean Theorem.Proof. Drop a perpendicular from C to ←→ AB and call its foot D. Label thesegments as shown in figure 38.2.By lemma 33.5, A ∗ D ∗ B.By the angle sum theorem 180 = m(∠A) + m(∠B) + m(∠C), hence since3 Mac Tutor History of Math, http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/Indian_sulbasutras.html.4 Internet Movie Data Base (IMDB), http://www.imdb.com/title/tt0032138/quotes?qt04099235 See Venema Problem 9.12.« CC BY-NC-ND 3.0. Revised: 18 Nov 2012

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