BOOK IV.

PROP. XV. PROB.

See N. To inscribe an equilateral and equiangular hexagon in a given circle.

f

Let ABCDEF be the given circle; it is required to inscribe an equilateral and equiangular hexagon in it.

Find the centre G of the circle ABCDEF, and draw the diameter AGD: and from D, as a centre, at the distance DG, describe the circle EGCH, join EG, CG, and produce them to the points B, F; and join AB, BC, CD, DE, EF, FA; The hexagon ABCDEF is equilateral and equiangular.

Because G is the centre of the circle ABCDEF, GE is equal to GD: And because D is the centre of the circle EGCH, DE is equal to DG; wherefore GE is equal to ED, and the triangle EGD is equilateral; and therefore its three angles EGD, GDE, DEG, are equal to one another, because the angles at the base of an isosceles 5. 1. triangle are equal; and the three angles of a triangle 32. 1. are equal to two right angles; therefore the angle EĞD is the third part of two right angles: In the same manner it may be demonstrated, that the angle DGC is also the third part of two right angles: And because the straight line GC makes with EB

b

the adjacent angles EGC, CGB 13. 1. equal to two right angles: the F remaining angle CGB is the third part of two right angles: therefore the angles EGD, DGC, CGB are equal to one another: And to E 15. 1. these are equal the vertical opposite angles BGA, AGF, FGE: Therefore the six angles EGD, DGC, CGB, BGA, AGF, FGE, are equal to one another: But 26. 3. equal angles stand upon equale

circumferences; therefore the six

H

B

circumferences AB, BC, CD, DE, EF, FA are equal to one another: And equal circumferences are sub29. 3. tended by equalf straight lines; therefore the six straight lines are equal to one another, and the hexagon ABCDEF is equilateral. It is also equiangular; for, since the cir

cumference AF is equal to ED, to each of these add the Book IV. circumference ABCD; therefore the whole circumference FABCD shall be equal to the whole EDCBA: And the angle FED stands upon the circumference FABCD, and the angle AFE upon EDCBA; therefore the angle AFE is equal to FED: In the same manner it may be demonstrated that the other angles of the hexagon ABCDEF are each of them equal to the angle AFE or FED: Therefore the hexagon is equiangular; and it is equilateral, as was shown; and it is inscribed in the given circle ABCDEF. Which was to be done.

COR. From this it is manifest, that the side of the hexagon is equal to the straight line from the centre, that is, to the semidiameter of the circle.

And if through the points A, B, C, D, E, F there be drawn straight lines touching the circle, an equilateral and equiangular hexagon shall be described about it, which may be demonstrated from what has been said of the pentagon; and likewise a circle may be inscribed in a given equilateral and equiangular hexagon, and circumscribed about it, by a method like to that used for the pentagon.

PROP. XVI, PROB.

To inscribe an equilateral and equiangular quin- See N. decagon in a given circle.

Let ABCD be the given circle; it is required to inscribe an equilateral and equiangular quindecagon in the circle ABCD.

Let AC be the side of an equilateral triangle inscribed a a 2. 4. in the circle, and AB the side of an equilateral and equiangular pentagon inscribed in the same; therefore, of such 11. 4. equal parts as the whole circumference ABCDF contains fifteen, the circumference ABC,

being the third part of the whole,
contains five; and the circumfer-
ence AB, which is the fifth part of B
the whole, contains three; there-
fore BC their difference contains E
two of the same parts: Bisect BC
in E; therefore BE, EC are, each
of them, the fifteenth part of the
whole circumference ABCD:

30. 3.

BOOK IV. Therefore, if the straight lines BE, EC be drawn, and straight lines equal to them be placed round d in the whole circle, an equilateral and equiangular quindecagon shall be inscribed in it. Which was to be done.

d 1. 4.

And in the same manner as was done in the pentagon, if, through the points of division made by inscribing the quindecagon, straight lines be drawn touching the circle, an equilateral and equiangular quindecagon shall be described about it: And, likewise, as in the pentagon, a circle may be inscribed in a given equilateral and equiangular quindecagon, and circumscribed about it.

THE

ELEMENTS

OF

EUCLID.

BOOK V.

DEFINITIONS.

I.

A LESS magnitude is said to be a part of a greater magni- Book V. tude, when the less measures the greater; that is, 'when

the less is contained a certain number of times exactly

' in the greater.'

II.

6

A greater magnitude is said to be a multiple of a less, when the greater is measured by the less; that is, when the greater contains the less a certain number of times exactly.

III.

'Ratio is a mutual relation of two magnitudes of the same See N. 'kind to one another, in respect of quantity.'

IV.

Magnitudes are said to have a ratio to one another, when the less can be multiplied so as to exceed the other.

V.

The first of four magnitudes is said to have the same ratio to the second, which the third has to the fourth, when any equimultiples whatsoever of the first and third being taken, and any equimultiples whatsoever of the second and fourth; if the multiple of the first be less than that of the second, the multiple of the third is also less than that of the fourth; or, if the multiple of the first be equal to that of the second, the multiple of the third is also equal to that of the fourth; or, if the multiple of the first

Book V.

be greater than that of the second, the multiple of the third is also greater than that of the fourth.

VI.

Magnitudes which have the same ratio are called proportionals.N. B. When four magnitudes are proportion'als, it is usually expressed by saying, the first is to the second, as the third to the fourth.'

VII.

When of the equimultiples of four magnitudes (taken as in the fifth definition), the multiple of the first is greater than that of the second, but the multiple of the third is not greater than the multiple of the fourth; then the first is said to have to the second a greater ratio than the third magnitude has to the fourth; and, on the contrary, the third is said to have to the fourth a less ratio than the first has to the second.

VIII.

Analogy, or proportion, is the similitude of ratios.

IX.

Proportion consists in three terms at least.

X.

When three magnitudes are proportionals, the first is said to have to the third the duplicate ratio of that which it has to the second..

XI.

See N. When four magnitudes are continual proportionals, the first is said to have to the fourth the triplicate ratio of that which it has to the second, and so on, quadruplicate, &c. increasing the denomination still by unity, in any number of proportionals.

Definition A, to wit, of compound ratio.

When there are any number of magnitudes of the same
kind, the first is said to have to the last of them the
ratio compounded of the ratio which the first has to the
second, and of the ratio which the second has to the
third, and of the ratio which the third has to the fourth,
and so on unto the last magnitude.
For example, if A, B, C, D, be four magnitudes of the same
kind, the first A is said to have to the last D the ratio
compounded of the ratio of A to B, and the ratio of B
to C, and the ratio of C to D, or, the ratio of A to D is
said to be compounded of the ratios of A to B, B to C,
and C to D: