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The other more simple cases may easily be deduced from this general case.
The converse is not proved by Euclid ; namely,-If two straight lines intersect one another, so that the rectangle contained by the parts of one is equal to the rectangle contained by the parts of the other; then a circle may be described passing through the extremities of the two lines. Or, in other words : -- If the diagonals of a quadrilateral figure intersect one another, so that the rectangle contained by the segments of one of them is equal to the rectangle contained by the segments of the other; then a circle may be described about the quadrilateral.
Prop. xxxvi. The converse of the corollary to this proposition may be thus stated :-If there be two straight lines, such that, when produced to meet, the rectangle contained by one of the lines produced, and the part produced, be equal to the rectangle contained by the other line produced and the part produced; then a circle can be described passing through the extremities of the two straight lines. Or, If two opposite sides of a quadrilateral figure be produced to meet, and the rectangle contained by one of the sides produced and the part produced, be equal to the rectangle contained by the other side produced and the part produced; then a circle may be described about the quadrilateral figure.
Prop. XXXVII. The demonstration of this theorem may be made shorter by a reference to the note on Euclid 111. Def. 2: for if DB meet the circle in B and do not touch it at that point, the line must, when produced, cut the circle in two points.
It is a circumstance worthy of notice, that in this proposition, as well as in Prop. xLvIII. Book 1. Euclid departs from the ordinary ex absurdo mode of proof of converse propositions.
QUESTIONS ON BOOK III. 1. DEFINE accurately the terms radius, arc, circumference, chord, secant.
2. How does a sector differ in form from a segment of a circle? Are they in any case coincident?
3. What is Euclid's criterion of the equality of two circles? What is meant by a given circle? How many points are necessary to determine the magnitude and position of a circle ?
4. When are segments of circles said to be similar ? Enunciate the propositions of the Third Book of Euclid, in which this definition is employed. Is it employed in a restricted or general form?
5. In how many points can a circle be cut by a straight line and by another circle ?
6. When are straight lines equally distant from the center of a circle? 7. Shew the necessity of an indirect demonstration in Euc. 111. 1.
8. Find the centre of a given circle without bisecting any straight line.
9. Shew that if the circumference of one of two equal circles pass through the center of the other, the portions of the two circles, each of which lies without the circumference of the other circle, are equal.
10. If a straight line passing through the center of a circle bisect a straight line in it, it shall cut it at right angles. Point out the exception; and shew that if a straight line bisect the arc and base of a segment of a circle, it will, when produced, pass through the center.
11. If any point be taken within a circle, and a right line be drawn from it to the circumference, how many lines can generally be drawn equal to it ? Draw them.
12. Find the shortest distance between a circle and a given straight line without it.
13. Shew that a circle can only have one center, stating the axioms upon which your proof depends.
14. Why would not the demonstration of Euc. 111. 9, hold good, if there were only two such equal straight lines ?
15. Two parallel chords in a circle are respectively six and eight inches in length, and one inch apart; how many inches is the diameter in length:
16. Which is the greater chord in a circle whose diameter is 10 inches; that whose length is 5 inches, or that whose distance from the center is 4 inches :
17. What is the locus of the middle points of all equal straight lines in a circle ?
18. The radius of a circle BCDGF, (fig. Euc. II. 15.) whose center is E, is equal to tive inches. The distance of the line FG from the center is four inches, and the distance of the line BC from the center is three inches, required the lengths of the lines FG, BC.
19. If the chord of an arc be twelve inches long, and be divided into two segments of eight and four inches by another chord: what is the length of the latter chord, if one of its segments be two inches?
20. What is the radius of that circle of which the chords of an arc and of double the arc are tive and eight inches respectively?
21. If the chord of an arc of a circle whose diameter is 8 inches, be five inches, what is the length of the chord of double the arc of the same circle?
22. State when a straight line is said to touch a circle, and shew from your definition that a straight line cannot be drawn to touch a circle from a point within it.
23. Can more circles than one touch a straight line in the same point?
24. Shew from the construction, Euc. 111. 17, that two equal straight lines, and only two, can be drawn touching a given circle from a given point without it: and one, and only one, from a point in the circumference.
25. What is the locus of the centers of all the circles which touch a straight line in a given point?
20. How may a tangint be drawn at a given point in the circumference of a circle, without knowing the center?
27. In a circle place two chords of given length at right angles to each other.
29. From Euc. m. 19, shew how many circles equal to a given circle mar be drawn to touch a siraight line in the same point.
19. Enunciate Eue: 11. 20. Is this true, when the base is greater than a semicircle? If so, why has Euclid omitted this case:
30. The angle at the center of a circle is double of that at the circumference. How will it appear hence that the angle in a semicircle is a right angles
31. What conditions are essential to the possibility of the inscription and circumscription of a circle in and about a quadrilateral tigure?
32. What conditions are requisite in order that a parallelogram may be inscribed in a circles Are there any analogous conditions requisite that a paraliclograun may be described about a circle ?
33. Detine the angle in a segment of a circle, and the angle on a seg.
ment; and shew that in the same circle, they are together equal to two right angles.
34. State and prove the converse of Euc. III. 22.
35. All circles which pass through two given points have their centers in a certain straight line.
36. Describe the circle of which a given segment is a part. Give Euclid's more simple method of solving the same problem independently of the magnitude of the given segment.
37. In the same circle equal straight lines cut off equal circumferences. If these straight lines have any point common to one another, it must not be in the circumference. Is the enunciation given complete?
38. Enunciate Euc. 111. 31, and deduce the proof of it from Euc. 111. 20.
39. What is the locus of the vertices of all right-angled triangles which can be described upon the same hypotenuse?
40. How may a perpendicular be drawn to a given straight line from one of its extremities without producing the line ?
41. If the angle in a semicircle be a right angle; what is the angle in a quadrant?
42. The sum of the squares of any two lines drawn from any point in a semicircle to the extremity of the diameter is constant. Express that constant in terms of the radius.
43. In the demonstration of Euc. III. 30, it is stated that “equal straight lines cut off equal circumferences, the greater equal to the greater, and the less to the less :" explain by reference to the diagram the meaning of this statement.
44. How many circles may be described so as to pass through one, two, and three given points? In what case is it impossible for a circle to pass through three given points ?
45. Compare the circunference of the segment (Euc. III. 33.) with the whole circumference when the angle contained in it is a right angle and a half.
46. Include the four cases of Euc. III. 35, in one general proof.
47. Enunciate the propositions which are converse to Props. 32, 35 of Book III.
48. If the position of the center of a circle be known with respect to a given point outside a circle, and the distance of the circumference to the point be ten inches: what is the length of the diameter of the circle, if a tangent drawn from the given point be fifteen inches?
49. If two straight lines be drawn from a point without a circle, and be both terminated by the concave part of the circumference, and if one of the lines pass through the center, and a portion of the other line intercepted by the circle, be equal to the radius : find the diameter of the circle, if the two lines meet the convex part of the circumference, a, b, units respectively from the given point.
50. Upon what propositions depends the demonstration of Euc. III. 35? Is any extension made of this proposition in the Third Book ?
51. What conditions must be fulfilled that a circle may pass through four given points ?
52. Why is it considered necessary to demonstrate all the separate cases of Euc. III, 35, 36, geometrically, which are comprehended in one formula, when expressed by Algebraic symbols ?
53. Enunciate the converse propositions of the Third Book of Euclid which are not demonstrated ex absurdo : and state the three methods which Euclid employs in the demonstration of converse propositions in the First and Third Books of the Elements.
PROPOSITION I. THEOREM. If AB, CD be chords of a circle at right angles to each other, prove that the sum of the arcs AC, BD is equal to the sum of the arcs AD, BC. Draw the diameter FGH parallel to AB, and cutting CD in H.
Then the arcs FDG and FCG are each half the circumference.
Also since CD is bisected in the point H,
the arc FD is equal to the arc FC, and the arc FD is equal to the arcs FA, AD, of which, AF is
equal to BG,
add to each CG, therefore the arcs AD, BC are equal to the arcs FC, CG, which make
up the half circumference. Hence also the arcs AC, DB are equal to half the circumference. Wherefore the arcs AD, BC are equal to the arcs AC, DB.
PROPOSITION II. PROBLEM.
The diameter of a circle having been produced to a given point, it is required to find in the part produced a point, from which if a tangent be drawn to the circle, it shall be equal to the segment of the part produced, that is, between the given point and the point found.
Analysis. Let AEB be a circle whose center is C, and whose diameter AB is produced to the given point D.
Suppose that G is the point required, such that the segment GD is equal to the tangent GE drawn from G to touch the circle in E.
Join DE and produce it to meet the circumference again in F;
join also CE and CF.
GEOMETRICAL EXERCISES ON BOOK 111.
and because CE is equal to CF,
the angle CEF is equal to the angle CFE; therefore the angles CEF, GED are equal to the angles CFE, GDE:
but since GE is a tangent at E,
hence the angles CEF, GEF are equal to a right angle, and consequently, the angles CFE, EDG are also equal to a right
angle, wherefore the remaining angle FCD of the triangle CFD is a right angle,
and therefore CF is perpendicular to AD. Synthesis. From the center C, draw CF perpendicular to AD meeting the circumference of the circle in F:
join DF cutting the circumference in E, join also CĚ, and at E draw EG perpendicular to CE and intersecting BD in G.
Then G will be the point required. For in the triangle CFD, since FCD is a right angle, the angles CFD, CDF are together equal to a right angle;
also since CEG is a right angle, therefore the angles CEF, GED are together equal to a right
angle; therefore the angles CEF, GED are equal to the angles CFD, CDF;
but because CE is equal to CF,
the angle CEF is equal to the angle CFD, wherefore the remaining angle GED is equal to the remaining
angle CDF, and the side G D is equal to the side GE of the triangle EGD, therefore the point G is determined according to the required • conditions.
PROPOSITION III. THEOREM. If a chord of a circle be produced till the part produced be equal to the radius, and if from its extremity a line be drawn through the center and meeting the conver and concave circumferences, the convex is one-third of the concave circumference.
Let AB any chord be produced to C, so that BC is equal to the radius of the circle :
and let CE be drawn from C through the center D, and meeting the convex circumference in F, and the concave in E.
Then the arc BF'is one-third of the arc AE.