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Ai a2 Az 24
and A,: A1 - Az:: A3: Az - ago Prop. xx. Algebraically. Let A1, A2, 4, be three magnitudes, and Q1, Q2, Qz, other three,
such that Aj : A2 :: 2 : Oh,
and A2 : A :: An : Ang :
A2 also since A2: Az :: A2 : Az, : .
Az and multiplying these equals,
41, A2 ai anz
Az and that Aj > A3:
It follows that ay is > Az. In the same way it may be shewn
that if A1 A3, then ay = az; and if A, be < A3, then an < Ago Prop. XXI. Algebraically.
Let A1, A2, A3, be three magnitudes,
and 21. az, as three others, such that 41 : 42 :: An : A3,
and 42: 43 :: Q : 02. If 4, > A3, then shall aj > az; and if equal, equal; and if less, less.
Multiplying these equals,
It follows that also Q > Az.
and if Aj < Az, also aj < Az.
Prop. XXII. Algebraically.
Let Aj, A2, A3 be three magnitudes,
and a2, A2, Az other three, such that Aį: A2 :: Q1: A2,
and A2 : A3 :: Az : Az. Then shall A1: 43 :: Qı : Az.
41 ay For since A1: A2 :: : Azn ..
and since A2: Az :: Az : 04,
Multiply these equals,
and A4: A2 :: a, : 23.
A, : A2 :: 21 : 22.
Az: A4 :: Az : 24:
and Aji A4 :: 2 : 04, and similarly, if there were more than four magnitudes. Prop. XXIII. Algebraically.
Let Aj, A2, A3 be three magnitudes
and aj, Az, az other three, such that A1 : A2 :: Az : A3,
and A2: Az :: : 22.
Az: A4 :: Q1 : 22 :.
:: A1: 44:: Qı : 24 and similarly, if there be more than four magnitudes. Prop. XXIV. Algebraically.
Let A, : Az :: 43 : 24,
and Ag : Oy :: Ag : 04,
24 Divide the former by the latter of these equals,
A6 : 24
A5 Az + A A6
aĄ and ... A + A, : Az :: Ag + A6 : . Cor. 1. Similarly may be shewn, that
41 – 45 : 22 :: 43 Prop. xxv. Algebraically.
Let A4: Az :: Az : 04,
Then shall 41 + 24 > Az + Az.
a4 Az - Az
A — Az A
Az - 84 Az
:: also A1 - A2 > Az - Q4,
.: 4, + 24 > Q2 + Az. “ The whole of the process in the Fifth Book is purely logical, that is, the whole of the results are virtually contained in the definitions, in the manner and sense in which metaphysicians (certain of them) imagine all the results of mathematics to be contained in their definitions and hypotheses. No assumption is made to determine the truth of any consequence of this definition, which takes for granted more about number or magnitude than is necessary to understand the definition itself. The
latter being once understood, its results are deduced by inspection-of itself only, without the necessity of looking at any thing else. Hence, a great distinction between the fifth and the preceding books presents itself. The first four are a series of propositions, resting on different fundamental assumptions; that is, about different kinds of magnitudes. The fifth is a definition and its developement; and if the analogy by which names have been given in the preceding Books had been attended to, the propositions of that Book would have been called corollaries of the definition.”—Connexion of Number and Magnitude, by Professor De Morgan, p.56.
The Fifth Book of the Elements as a portion of Euclid's System of Geometry ought to be retained, as the doctrine contains some of the most important characteristics of an effective instrument of intellectual Education. This opinion is favoured by Dr. Barrow in the following expressive terms: “There is nothing in the whole body of the Elements of a more subtile invention, nothing more solidly established, or more accurately handled than the doctrine of proportionals.'
QUESTIONS ON BOOK V.
1. EXPLAIN and exemplify the meaning of the terms, multiple, submultiple, equimultiple.
2. What operations in Geometry and Arithmetic are analogous ?
3. What are the different meanings of the term measure in Geometry? When are Geometrical magnitudes said to have a common measure ?
4. When are magnitudes said to have, and not to have, a ratio to one another? What restriction does this impose upon the magnitudes in regard to their species ?
5. When are magnitudes said to be commensurable or incommensurable to each other? Do the definitions and theorems of Book v, include incommensurable quantities ?
6. What is meant by the term geometrical ratio? How is it represented ? 7. Why does Euclid give no independent definition of ratio ?
8. What sort of quantities are excluded from Euclid's idea of ratio, and how does his idea of ratio differ from the Algebraic definition?
9. How is a ratio represented Algebraically? Is there any distinction between the terms, a ratio of equality, and equality of ratio?
10. In what manner are ratios, in Geometry, distinguished from each other as equal, greater, or less than one another? What objection is there to the use of an independent definition (properly so called) of ratio in a system of Geometry?
11. Point out the distinction between the geometrical and algebraical methods of treating the subject of proportion.
12. What is the geometrical definition of proportion? Whence arises the necessity of such a definition as this?
13. Shew the necessity of the qualification " any whatever”' in Euclid's definition of proportion.
14. Must magnitudes that are proportional be all of the same kind ? 15. To what objection has Euc. v. def. 5, been considered liable ?
16. Point out the connexion between the more obvious definition of proportion and that given by Euclid, and illustrate clearly the nature of the advantage obtained by which he was induced to adopt it.
17. Why may not Euclid's definition of proportion be superseded in