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Book I. Let the straight line AB make with CD, upon one side of it, the Mangles CBA, ABD; these are either two right angles, or are toge
ther equal to two right angles. a. Def. 10. For if the angle CBA be equal to ABD, each of them is a right
d. 1. Ax.
B angle. but if not, from the point B draw BE at right angles b to CD. therefore the angles CBE, EBD are two right anglesa, and because CBE is equal to the two angles CBA, ABE together ; add the angle
EBD to each of these equals, therefore the angles CBE, EBD are e. 2. As. equal to the three angles CBA, ABE, EBD, again, because the e
angle DBA is equal to the two angles DBE, EBA, add to these e-
PROP. XIV. THEOR.
upon the opposite sides of it, make the adjacent angles together equal to two right angles, these two straight lines shall be in one and the same straight line.
At the point B in the straight
D straight line with CB, let BE be
in the fame straight line with it. therefore because the straight line Book I. AB makes angles with the straight line CBE, uport one side of it, the angles ABC, ABE are together equal'to two right angles; but the 2. 13. 1. angles ABC, ABD are likewise together equal to two right angles; therefore the angles CBA, ABE are equal to the angles CBA, ABD: take away the common angle ABC; the remaining angle ABE is equal b to the remaining angle ABD, the less to the greater, which b. 3. Ax. is impossible. therefore BE is not in the same straight line with BC: And in like manner, it may be demon/trated that no other can be in the same straight line with it but BD, which therefore is in the fame straight line with CB. Wherefore if at a point, &c. Q: E. D.
F two straight lines cut one another, the vertical, or opposite, angles shall be equal.
2. 13. I
Let the two straight lines AB, CD cut one another in the point E. the angle AEC shall be equal to the angle DEB, and CEB to AED.
Because the straight line AE makes with CD the angles CEA, AED, these angles åre together equalto two right angles. again, because the straight line DE makes with AB the angles AED, DEB; these also are together equal to two right angles. and A E
B CEA, AED have been demonstrated to be equal to two right
D angles; wherefore the angles CEA, AED are equal to the angles AED, DEB. take away the common angle AED, and the remaining angle CEA is equal o b. 3. Ax. to the remaining angle DEB. In the fame manner it can be demonstrated that the angles CEB, AED are equal. therefore if two Itraight lines, &c. Q. E. D.
Cor. 1. From this it is manifest that if two straight lines cut one another, the angles they make at the point where they cut, are together equal to four right angles.
Cor. 2. And consequently that all the angies made by any number of lines meeting in one point, are together equal to four right angles.
6. 4. I.
PROP. XVI. THEOR.
angle is greater than either of the interior opposite angles.
Let ABC be a triangle, and let its fide BC be produced to D.
Because AE is equal to
C b. 15. 1.
is equal b to the angle CEF,
ner, if the side BC be bisected, it may be demonstrated that the d. 15. 1. angle BCG, that is, the angle ACD, is greater than the angle ABC. therefore if one side, &c. Q. E. D.
PROP. XVII. THEOR.
two right angles.
Produce BC to D; and be.
of the triangle ABC, ACD is 4. 1$. 1. greater
* than the interior and opposite angle ABC; to each of B C D
19 these add the angle ACB, therefore the angles ACD, ACB are great. Book I. er than the angles ABC, ACB. but ACD, ACB are together equalb m to two right angles; therefore the angles ABC, BCA are less than b. 13. t. two right angles. in like manner it may be demonstrated that BAC, ACB, as also CAB, ABC are less than two right angles. therefore any two angles, &c. Q. E. D.
THE greater side of
every triangle is opposite to the
a. j. t, and join BD, and because ADB
B is the exterior angle of the triangle BDC, it is greater than the interior and opposite angle b. 16. 1: DCB. but ADB is equal to ABD, because the side AB is e- c. 5. 1. qual to the side AD; therefore the angle ABD is likewise greater than the angle ACB; wherefore much more is the angle ABC greattr than ACB. therefore the greater fide, &c. Q. E. D.
PROP. XIX. THEOR.
the greater side, or has the greater side opposite to it. Let ABC be a triangle of which the angle ABC is greater than the angle BCA. the fide AC is likewise greater than the fide AB. For if it be not greater, AC
А must either be equal to AB, or less than it. it is not equal, because then the angle ABC would be equal to the angle ACB; but it
&. $.fi is not; therefore AC is not equal to AB. neither is it less; because B then the angle ABC would be less B2
Book I. b than the angle ACB; but it is not; therefore the side AC is not
mless than AB. and it has been shewn that it is not equal to AB. b 18.1. therefore AC is greater than AB. wherefore the greater angle, &c,
Q. E. D.
a. 3. 1
NY two sides of a triangle are together greater than
the third side. Let ABC be a triangle; any two sides of it together are greater than the third fide, viz. the sides BA, AC greater than the side BC; and AB, BC greater than AC; and BC, CA greater than AB. Produce BA to the point D,
Because DA is equal to AC,
c. 19. 1.
F from the ends of the side of a triangle there be drawn
shall be less than the other two sides of the triangle, but thall contain a greater angle.
Let the two straight lines BD, CD be drawn from B, C, the ends of the side BC of the triangle ABC, to the point D within it. BD and DC are less than the other two sides BA, AC of the triangle, but contain an angle BDC greater than the angle BAC.
Produce BD to E; and because two sides of a triangle are greater than the third side, the two sides BA, AE of the triangle ABE