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(26. 1.); wherefore DE is equal to DF: for the same reason, DG is equal to DF : therefore the three straight lines DE, DF, DG are equal to one another, and the circle described from the centre D, at the distance of any of them, shall pass through the extremities of the other two, and touch the straight lines AB, BC, CA, because the angles at the points E, F, G are right angles, and the straight line which is drawn from the extremity of a diameter at right angles to it, touches (16. 3.) the circle : therefore the straight lines AB, BC, CA do each of the n touch the circle, and the circle EFG is inscribed in the triangle ABC. Which was to be done.

PROP. V. PROB.

To describe a circle about a given triangle. *

Let the given triangle be ABC; it is required to describe a circle about ABC.

Bisect (10. 1.) AB, AC in the points D, E, and from these points draw DF, EF at right angles (11. 1.) to AB, AC ; DF, А A

A

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EF produced meet one another: for, if they do not meet, they are parallel, wherefore AB, AC, which are at right angles to them, are parallel ; which is absurd : let them meet in F, and join FA; also, if the point F be not in BC, join BF, CF; then, because AD is equal to DB, and DF common, and at right angles to AB, the base AF is equal (4. 1.) to the base FB : in like manner, it may be shown, that CF is equal to FA ; and therefore BF is equal to FC; and FA, FB, FC are equal to one another;

* See Note.

wherefore the circle described from the centre F, at the distance of one of them, shall pass through the extremities of the other two, and be described about the triangle ABC. Which was to be done.

COR. And it is manifest, that when the centre of the circle falls within the triangle, each of its angles is less than a right angle, each of them being in a segment greater than a semicirele ; but, when the centre is in one of the sides of the triangle, the angle opposite to this side, being in a semicircle, is a right angle; and, if the centre falls without the triangle, the angle opposite to the side beyond which it is, being in a segment less than a semicircle, is greater than a right angle : wherefore, if the given triangle be acate angled, the centre of the circle falls within it; if it be a right angled triangle, the centre is in the side opposite to the right angle; and, if it be an obtuse angled triangle, the centre falls without the triangle, beyond the side opposite to the obtuse angle.

PROP. VI. PROB.

To inscribe a square in a given circle.

Let ABCD be the given circle ; it is required to inscribe a square in ABCD.

Draw the diameters AC, BD at right angles to one another ; and join AB, BC, CD, DA; because BE is equal to ED, for E is the centre, and that EA is com

А mon, and at right angles to BD; the base BA is equal (4. 1.) to the base AD; and for the same reason, BC, CD are

E each of them equal to BA or AD; B

D therefore the quadrilateral figure ABCD is equilateral. It is also rectangular ; for the straight line BD, being the diameter of the circle ABCD, BAD is a

С semicircle ; wherefore the angle BAD is a right (31. 3.) angle ; for the same reason each of the angles ABC, BCD, CDA is a right angle; therefore the quadrilateral figure ABCD is rectangular, and it has been shown to be equilateral; therefore it is a square ; and it is inscribed in the circle ABCD. Which was to be done.

PROP. VII. PROB.

To describe a square about a given circle. Let ABCD be the given cirelė : it is required to describe a

square about it.

Draw two diameters AC, BD, of the circle ABCD, at right angles to one another, and through the points A, B, C, D, draw (17. 3.) FG, GH, HK, KF touching the circle; and because FG touches the circle ABCD, and EA is drawn from the centre E to the point of contact A, the angles at A are right (18. 3.) angles ; for the same reason, the angles at the points B, C, D are right angles; and because the angle AEB is G A F a right'angle, as likewise is EBG, GH is parallel (28. 1.) to. AC; for the same reason, AC is parallel to FK, and in

E like manner GF, HK may each of them B

D be demonstrated to be parallel to BED; therefore the figures GK, GC, AK, FB, BK are parallelograms; and GF is therefore equal (34. 1.) to HK, and GH

H С

K to FK; and because AC is equal to BD, and that AC is equal to each of the two GH, FK; and BD to each of the two GF, HK: GH, FK are each of them equal to GF or HK ; therefore the quadrilateral figure FGHK is equilateral. It is also rectangular: for GBEA being a parallelogram, and AEB a right angle, AGB (34. 1.) is likewise a right angle: in the same manner, it may be shown that the angles at H, K, F are right angles ; therefore the quadrilateral figure FGHK is rectangular, and it was demonstrated to be equilateral : therefore it is a square; and it is described about the circle ABCD. Which was to be done.

PROP. VIII. PROB.

To inscribe a circle in a given square.

Let ABCD be the given square ; it is required to inscribe a circle in ABCD.

Bisect (10. 1.) each of the sides AB, AD, in the points F, E, and through E draw (31. 1.) EH parallel to AB or DC, and through

F draw FK parallel to AD or BC; therefore each of the figures AK, KB, AH, HD, AG, GC, BG, GD is a parallelogram, and their opposite sides are equal (34. 1.); and because AD is equal to AB, and that AE is the half of AD, and AF the half of AB, AE is equal to AF; wherefore the sides A E D opposite to these are equal, viz. FG to GE: in the same manner, it may be demonstrated that GH, GK are each of

G them equal to FG or GE; therefore F

K the four straight lines GE, GF, GH, GK, are equal to one another; and the circle described from the centre G, at the distance of one of them, shall pass

в н through the extremities of the other three, and touch the straight lines AB, BC, CD, DA; because the angles at the points E, F, H, K are right (29. 1.) angles, and that the straight line which is drawn from the extremity of a diameter, at right angles to it, touches the circle (16. 3.); therefore each of the straight lines AB, BC, CD, DA touches the circle, which therefore is inscribed in the square ABCD. Which was to be done.

PROP. IX. PROB.

To describe a circle about a given square.

E

Let ABCD be the given square; it is required to describe a circle about it.

Join AC, BD cutting one another in E; and because DA is equal to AB, and AC common to the triangles DAC, BAC, the two sides DA, AC are equal to the two BA,

A AC; and the base DC is equal to the base

D BC; wherefore the angle DAC, is equal (8. 1.) to the angle BAC, and the angle DAB is bisected by the straight line AC: in the same manner, it may be demonstrated that the angles ABC, BCD, CDA are severally bisected by the straight lines BD, AC; there B В fore, because the angle DAB is equal to the angle ABC, and that the angle EAB is the half of DAB, and EBA the half of ABC ; the angle EAB is equal to the angle EBA ; wherefore the side EX is equal (6. 1.) to the side EB : in the same manner, it may be demonstrated that

the straight lines EC, ED are each of them equal to EA or EB: therefore the four straight lines EA, EB, EC, ED are equal to one another; and the circle described from the centre E, at the distance of one of them, shall pass through the extremities of the other three, and be described about the square ABCD. Which was to be done.

PROP. X. PROB.

To describe an isosceles triangle, having each of the angles at the base double of the third angle.

Take any straight line AB, and divide (11. 2.) it in the point C, so that the rectangle AB, BC be equal to the square of CA; and from the centre A, at the distance AB, describe the circle BDE, in which place (1. 4.) the straight line BD is equal to AC, which is not greater than the diameter of the circle BDE; join DA, DC, and about the triangle ADC describe (5. 4.) the circle ACD; the triangle ABD is such as is required, that is, each of the angles ABD, ADB is double of the angle BAD.

Because the rectangle AB, BC is equal to the square of AC, and that AC is equal to BD, the rectangle AB, BC is equal to the square of BD; and because

E from the point B without the circle ACD two straight lines BCA, BD are drawn to the circumference, one of which cuts, and the other meets the circle,

A and that the rectangle AB, BC contained by the whole of the cutting line, and the part of it without the circle is equal to the

C square of BD which meets it; the straight line BD touches (37. 3.) the circle ACD; and because BD touches the circle, and DC is drawn from the point of con

B

D tact D, the angle BDC is equal (32. 3.) to the angle DAC in the alternate segment of the circle; to each of these add the angle CDA: therefore the whole angle BDA is equal to the two angles CDA, DAC; but the exterior angle BCD is equal (32. 1.) to the angles CDA, DAC; therefore also BDA is equal to BCD;

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