Proposition XXV. Theorem.
|(104)||If two triangles (B A C and E F D) have two sides of the one respectively equal to two of the other (B A to E F and A C to F D), and if the third side of the one (B C) be greater than the third side (E D) of the other, the angle (A) opposite the greater side is greater than the angle (F), which is opposite to the less.|
The angle A is either equal to the angle F, or less than it, or greater than it.
It is not equal; for if it were, the side B C would be equal to the side E D (IV), which is contrary to the hypothesis.
It is not less; for if it were, the side B C would be less than the side E D (XXIV), which is contrary to the hypothesis.
Since therefore the angle A is neither equal to, nor less than F, it must be greater.
This proposition might be proved directly thus: On the greater side B C take B G equal to the lesser side E D, and on B G construct a triangle B H G equilateral with E F D. Join A H and produce H G to I.
The angle H will then be equal to the angle F.
1° Let B G be greater than B K.
Since B A and B H are equal, the angles B A H and B H A are equal (V). Also since H G is equal to A C, it is greater than A I, and therefore H I is greater than A I, and therefore the angle H A I is greater than the angle A H I (XVIII). Hence, if the equal triangles B H A and B A H be added to these, the angle B A C will be found greater than the angle B H G, which is equal to F.
2° If B G be not greater than B K, it is evident that the angle H is less than the angle A.
The twenty-fourth and twenty-fifth propositions are analogous to the fourth and eighth, in the same manner as the eighteenth and nineteenth are to the fifth and sixth . The four might be announced together thus:
If two triangles have two sides of the one respectively equal to two sides of the other, the remaining side of the one will be greater or less than, or equal to the remaining side of the other, according as the angle opposed to it in the one is greater or less than, or equal to the angle opposed to it in the other, or vice versa.
In fact, these principles amount to this, that if two lines of given lengths be placed so that one pair of extremities coincide, and so that in their initial position the lesser line is placed upon the greater, the distance between the extremities will then be the difference of the lines. If they be opened as to form a gradually increasing angle, the line joining their extremities will gradually increase, until the angle they include becomes equal to two right angles, when they will be in one continued line, and the line joining their extremities is their sum. Thus the major and minor limits of this line is the sum and difference of the given lines. This evidently includes the twentieth proposition.
Book I: Euclid, Book I (ed. Dionysius Lardner, 11th Edition, 1855)
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