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is defined to be the converse of another when the hypothesis of the former becomes the predicate of the latter ; and vice versa.
There is besides this, another kind of conversion, when a theorem has several hypotheses and one predicate; by assuming the predicate and one, or more than one of the hypotheses, some one of the hypotheses may be inferred as the predicate of the converse. In this manner, Prop. viii. is the converse of Prop. iv. It may here be observed, that converse theorems are not universally true: as for instance, the following direct proposition is universally true; “If two triangles have their three sides respectively equal, the three angles of each shall be respectively equal." But the converse is not universally true; namely, “If two triangles have the three angles in each respectively equal, the three sides are respectively equal.” Converse theorems require, in some instances, the consideration of other conditions than those which enter into the proof of the direct theorem. Converse and contrary propositions are by no means to be confounded; the contrary proposition denies what is asserted, or asserts what is denied, in the direct proposition, but the subject and predicate in each are the same. A contrary proposition is a completely contradictory proposition, and the distinction consists in this—that two contrary propositions may both be false, but of two contradictory propositions, one of them must be true, and the other false. It may here be remarked, that one of the most common intellectual mistakes of learners, is to imagine that the denial of a proposition is a legitimate ground for affirming the contrary as true : whereas the rules of sound reasoning allow that the affirmation of a proposition as true, only affords a ground for the denial of the contrary as false.
Prop. vil is the first instance of indirect demonstrations, and they are more suited for the proof of converse propositions. All those propositions which are demonstrated ex absurdo, are properly analytical demonstrations, according to the Greek notion of analysis, which first supposed the thing required, to be done, or to be true, and then shewed the consistency or inconsistency of this construction or hypothesis with truths admitted or already demonstrated.
In indirect demonstrations, where hypotheses are made which are not true and contrary to the truth stated in the proposition, it seems desirable that a form of expression should be employed different from that in which the hypotheses are true. In all cases therefore, whether noted by Euclid or not, the words if possible have been introduced, or some such qualifying expression, as in Euc. 1. 6, so as not to leave upon the mind of the learner, the impression that the hypothesis which contradicts the proposition, is really true.
Prop. VIII. When the three sides of one triangle are shewn to coincide with the three sides of any other, the equality of the triangles is at once obvious. This, however, is not stated at the conclusion of Prop. VIII. or of Prop. XXVI. For the equality of the areas of two coincident triangles, reference is always made by Euclid to Prop. IV.
A direct demonstration may be given of this proposition, and Prop. VII. may be dispensed with altogether.
Let the triangles ABC, DEF be so placed that the base BC may coincide with the base EF, and the vertices A, D may be on opposite sides of EF. Join AD. Then because EAD is an isosceles triangle, the angle EAD is equal to the angle EDA; and because CDA is an isosceles triangle, the angle CAD is equal to the angle CDA. Hence
the angle EAP is equal to the angle EDP, (ax. 2 or 3): or the angle BDC is equal to the angle EDP.
Prop. ix. If BA, AC be in the same straight line. This problem then becomes the same as Prob. XI, which may be regarded as drawing a line which bisects an angle equal to two right angles.
If FA be produced in the fig. Prop. 9, it bisects the angle which is the defect of the angle BAC from frur right angles.
By means of this problem, any angle may be divided into four, eight, sixteen, &c. equal angles.
Prop. x. A finite straight line may, by this problem, be divided into four, eight, sixteen, &c. equal parts
Prop. XI. When the point is at the extremity of the line; by the second postulate the line may be produced, and then the construction applies. See note on Euc. 111. 31.
The distance between two points is the straight line which joins the points; but the distance between a point and a straight line, is the shortest line which can be drawn from the point to the line.
From this Prop. it follows that only one perpendicular can be drawn from a given point to a given line; and this perpendicular may be shewn to be less than any other line which can be drawn from the given point to the given line: and of the rest, the line which is nearer to the perpendicular is less than one more remote from it: also only two equal straight lines can be drawn from the same point to the line, one on each side of the perpendicular or the least. This property is analogous to Euc. III. 7, 8.
The corollary to this proposition is not in the Greek text, but was added by Simson, who states that it “is necessary to Prop. 1, Book XI., and otherwise.”
Prop. XII. The third postulate requires that the line CD should be drawn before the circle can be described with the center C, and radius CD.
Prop. xlv. is the converse of Prop. XIII. “Upon the opposite sides of it." "If these words were omitted, it is possible for two lines to make with a third, two angles, which together are equal to two right angles, in such a manner that the two lines shall not be in the same straight line.
The line Be may be supposed to fall above, as in Euclid's figure, or below the line BD, and the demonstration is the same in form.
Prop. xv. is the development of the definition of an angle. If the lines at the angular point be produced, the produced lines have the same incli. nation to one another as the original lines, but in a different positioil.
The converse of this Proposition is not proved by Euclid, namely:If the vertical angles made by four straight lines at the same point be respectively equal to each other, each pair of opposite lines shall be in the same straight line.
Prop. xvii. appears to be only a corollary to the preceding proposition, and it seems to be introduced to explain Axiom xii, of which it is the converse The exact truth respecting the angles of a triangle is proved in Prop. XXXII.
Prop. XVIII. It may here be remarked, for the purpose of guarding the student against a very common mistake, that in this proposition and in the converse of it, the hypothesis is stated before the predicate.
Prop. xix. is the converse of Prop. XVII. It may be remarked, that Prop. xix. bears the same relation to Prop. XVIII., as Prop. VI. does to Prop. v.
Prop. xx. The following corollary arises from this proposition :
A straight line is the shortest distance between two points. For the straight line BC is always less than BA and AC, however near the point 4 may be to the line BC.
It may be easily shewn from this proposition, that the difference of any two sides of a triangle is less than the third side.
Prop. XXII. When the sum of two of the lines is equal to, and when it is less than, the third line; let the diagrams be described, and they will exhibit the impossibility implied by the restriction laid down in the Proposition.
The same remark may be made here, as was made under the first Proposition, namely:- if one circle lies partly within and partly without another circle, the circumferences of the circles intersect each other in two points.
Prop. XXIII. CD might be taken equal to CE, and the construction effected by means of an isosceles triangle. It would, however, be less general than Euclid's, but is more convenient in practice.
Prop. xxiv. Simson makes the angle EDG at D in the line ED, the side which is not the greater of the two ED, DF; otherwise, three different cases would arise, as may be seen by forming the different figures. The point G might fall below or upon the base Ef produced as well as above it. Prop. xxiv. and Prop. xxv. bear to each other the same relation as Prop. IV. and Prop. VIII.
Prop. xxvI. This forms the third case of the equality of two triangles. Every triangle has three sides and three angles, and when any three of one triangle are given equal to any three of another, the triangles may be proved to be equal to one another, whenever the three magnitudes given in the hypothesis are independent of one another. Prop. IV. contains the first case, when the hypothesis consists of two sides and the included angle of each triangle. Prop. VIII. contains the second, when the hypothesis consists of the three sides of each triangle. Prop. XXVI. contains the third, when the hypothesis consists of two angles, and one side either adjacent to the equal angles, or opposite to one of the equal angles in each triangle. There is another case, not proved by Euclid, when the hypothesis consists of two sides and one angle in each triangle, but these not the angles included by the two given sides in each triangle. This case however is only true under a certain restriction, thus:
If two triangles have two sides of one of them equal to two sides of the other, each to each, and have also the angles opposite to one of the equal sides in each triangle, equal to one another, and if the angles opposite to the other equal sides be both acute, or both obtuse angles; then shall the third sides be equal in each triangle, as also the remaining angles of the one to the remaining angles of the other.
Let ABC, DEF be two triangles which have the sides AB, AC equal to the two sides DE, DF, each to each, and the angle ABC equal to the angle DEF: then, if the angles ACB, DEF, be both acute, or both obtuse anyles, the third side BC shall be equal to the third side EF, and also the angle BCA to the angle EFD, and the angle BAC to the angle EDF.
First. Let the angles ACB, DFE opposite to the equal sides AB, DE, be both acute angles.
If BC be not equal to EF, let BC be the greater, and from BC, cut off BG equal to EF, and join AG.
Then in the triangles ABG, DEF, Euc, I. 4. AG is equal to DF,
and the angle AGB to Dye. But since AC is equal to DF, AG is equal to AC: and therefore the angle ACG is equal to the angle AGC, which is also an acute angle. But because AGC, AGB are together equal to two right angles, and that AGC is an acute angle, AGB must be an obtuse angle; which is absurd. Wherefore, BC is not unequal to Er, that is, BC is equal to Ef, and also the remaining angles of one triangle to the remaining angles of the other.
Secondly. Let the angles ACB, DFE, be both obtuse angles. By proceeding in a similar way, it may be shewn that BC cannot be otherwise than equal to EF.
If ACB, DFE be both right angles: the case falls under Euc. 1. 26.
Prop. XXVII. Alternate angles are defined to be the two angles which two straight lines make with another at its extremities, but upon opposite sides of it.
When a straight line intersects two other straight lines, two pairs of alternate angles are formed by the lines at their intersections, as in the figure, BEF, EFC are alternate angles as well as the angles AEF, EFD.
Prop. xxvi. One angle is called “ the exterior angle,” and another “the interior and opposite angle," when they are formed on the same side of a straight line which falls upon or intersects two other straight lines. It is also obvious that on each side of the line, there will be two exterior and two interior and opposite angles. The exterior angle EGB has the angle GHD for its corresponding interior and opposite angle: also the exterior angle FHD has the angle HGB for its interior and opposite angle.
Prop. xxix is the converse of Prop. XXVII and Prop. XXVIII.
As the definition of parallel straight lines simply describes them by a statement of the negative property, that they never meet; it is necessary that some positive property of parallel lines should be assumed as an axiom, on which reasonings on such lines may be founded.
Euclid has assumed the statement in the twelfth axiom, which has been objected to, as not heing self-evident. A stronger objection appears to be, that the converse of it forms Euc. 1. 17; for both the assumed axiom and its converse, should be so obvious as not to require formal demonstration.
Simson has attempted to overcome the objection, not by any improved definition and axiom respecting parallel lines; but, by considering Euclid's twelfth axiom to be a theorem, and for its proof, assuming two definitions and one axiom, and then demonstrating five subsidiary Propositions.
Instead of Euclid's twelfth axiom, the following has been proposed as a more simple property for the foundation of reasonings on parallel lines; namely, “If a straight line fall on two parallel straight lines, the alternate angles are equal to one another.” In whatever this may exceed Euclid's definition in simplicity, it is liable to a similar objection, being the converse of Euc. 1. 27.
Professor Playfair has adopted in his Elements of Geometry, that “Two straight lines which intersect one another cannot be both parallel to the same straight line. This apparently more simple axiom follows as a direct inference from Euc. 1. 30.
But one of the least objectionable of all the definitions which have been proposed on this subject, appears to be that which simply expresses the conception of equidistance. It may be formally stated thus : “Parallel lines are such as lie in the same plane, and which neither recede from, nor approach to, each other." This includes the con
ception stated by Euclid, that parallel lines never meet. Dr. Wallis observes on this subject, “Parallelismus et æquidistantia vel idem sunt, vel certe se mutuo comitantur."
As an additional reason for this definition being preferred, it may be remarked that the meaning of the terms ypumpai napálinkoi, suggests the exact idea of such lines.
An account of thirty methods which have been proposed at different times for avoiding the difficulty in the twelfth axiom, will be found in the appendix to Colonel Thompson's “Geometry without Axioms.”
Prop. xxx. In the diagram, the two lines AB and CD are placeil one on each side of the line EF: the proposition may also be proved when both AB and CD are on the same side of EF.
Prop. xxxi. From this proposition, it is obvious that if one angle of a triangle be equal to the sum of the other two angles, that angle is a right angle, as is shewn in Euc. Ill. 31, and that each of the angles of an equilateral triangle, is equal to two thirds of a right angle, as it is shewn in Euc. iv, 15. Also, if one angle of an isosceles triangle be a right angle, then each of the equal angles is half a right angle, as in Euc. 11. 9.
The three angles of a triangle may be shewn to be equal to two right angles without producing a side of the triangle, by drawing through any angle of the triangle a line parallel to the opposite side, as Proclus has remarked in his Commentary on this proposition. It is manifest from this proposition, that the third angle of a triangle is not inde. pendent of the sum of the other two; but is known if the sum of any two is known. Cor. 1 may be also proved by drawing lines from any one of the angles of the figure to the other angles. If any of the sides of the figure bend inwards and form what are called re-entering angles, the enunciation of these two corollaries will require some modification. As Euclid gives no definition of re-entering angles, it may fairly be concluded, he did not intend to enter into the proofs of the properties of figures which contain such angles.
Prop. XXXIII. The words “towards the same parts” are a necessary restriction: for if they were omitted, it would be doubtful whether the extremities A, C, and B, D were to be joined by the lines AC and BD; or the extremities A, D, and B, C, by the lines AD and BC.
Prop. xxxiv. If the other diameter be drawn, it may be shewn that the diameters of a parallelogram bisect each other, as well as bisect the area of the parallelogram. If the parallelogram be right angled, the diagonals are equal; it the parallelogram be a square or a rhombus, the diagonals bisect each other at right angles. The converse of this Prop., namely, “ If the opposite sides or opposite angles of a quadrilateral figure be equal, the opposite sides shall also be parallel ; that is, the figure shall be a parallelogram," is not proved by Euclid.
Prop. xxxv. The latter part of the demonstration is not expressed very intelligibly. Simson, who altered the demonstration, seems in fact to consider two trapeziums of the same form and magnitude, and from one of them, to take the triangle ABE; and from the other, the triangle DCF; and then the remainders are equal by the third axiom : that is, the parallelogram ABCD is equal to the parallelogram EBCF. Otherwise, the triangle, whose base is DE, (fig. 2.) is taken twice from the trapezium, which would appear to be impossible, if the sense in which Euclid applies the third axiom, is to be retained here.