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PROP. XII. PROB.
To draw a straight line perpendicular to a given straight line, of an
unlimited length, from a given point without it.
Let AB be a given straight line, which may be produced to any length both ways, and let C be a point without it. It is required to draw a straight line perpendi
C cular to AB from the point C. Take any point D upon the
E other side of AB, and from the centre C, at the distance CD, describe (3. Post.) the circle EGF meeting AB in F,G; and A E H
B bisect (10. 1.) FG in H, and
D join CF, CH, CG; the straight line CH, drawn from the given point C, is perpendicular to the given straight line AB.
Because FH is equal to HG, and HC common to the two triangles FHC, GHC, the two sides FH, HC are equal to the two GH, HC, each to each ; but the base CF is also equal (11. Def. 1.) to the base CG; therefore the angle CHF is equal (8. 1.) to the angle CHG; and they are adjacent angles ; now when a straight line standing on a straight line makes the adjacent angles equal to one another, each of them is a right angle, and the straight line which stands upon the other is called a perpendicular to it; therefore from the given point C a perpendicular CH has been drawn to the given straight line AB. Which was to be done.
PROP. XIII. THEOR.
The angles which one straight line makes with another upon one side of
it, are either two right angles, or are together equal to two right angles.
Let the straight line AB make with CD, upon one side of it the angles CBA, ABD; these are either two right angles, or are together equal to two right angles.
For, if the angle CBA be equal to ABD, each of them is a right apgle (Def. 7.) ; but, if not, from the point B draw BE at right an
gles (11. 1.) to CD; therefore the angles CBE, EBD are two right angles. Now, the angle CBE is equal to the two angles CBA, ABE together ; add the angle EBD to each of these equals, and the two angles CBE, EBD, will be equal (2. Ax.) to the three CBA, ABE, EBD. Again, the angle DBA is equal to the two angles DBE, EBA ; add to each of these equals the apgle ABC ; then will the two angles DBA, ABC be equal to the three angles DBE, EBA, ABC; but the angles CBE, EBD have been demonstrated to be equal to the same three angles ; and things that are equal to the same are equal (1. Ax.)
; to one another; therefore the angles CBE, EBD are equal to the angles DBA, ABC; but CBE, EBD, are two right angles; therefore DBA, ABC are together equal to two right angles. Wherefore, when a straight line, &c. Q. E. D.
PROP. XIV. THEOR.
If, at a point in a straight line, two other straight lines, upon the opposite sides of it, make the adjacent angles together equal to two right angles, these two straight lines are in one and the same straight line. At the point B in the straight
A line AB, let the two straight lines BC, BD upon the opposite sides of AB, make the adjacent angles ABC, ABD equal together to two right angles. BD is in the same straight line with CB. For if BD be not in the same
E straight line with CB, let BE be in the same straight line with it ;
D therefore, because the straight line AB makes angles with the straight line CBE, upon one side of it, the angles ABC, ABE are together equal (13. 1.) to two right angles ; but the 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, and the remain. ing angle ABE is equal (3. Ax.) to the remaining angle ABD, the less to the greater, which is impossible ; therefore BE is not in the same straight line with BC. And in like manner, it
be demonstrated, that no other can be in the same straight line with it but BD, which therefore is in the same straight line with CB. Wherefore, if at a point, &c. Q. E. D.
PROP. XV. THEOR.
If two straight lines cut one another, the vertical, or opposite angles are
equal. 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.
For the angles CEA, AED, which the straight line AE makes with the straight line CD, are together equal (13. 1.) to two right angles; and the angles AED, DEB,
C which the straight line DE makes with the straight line AB, are also together equal (13. 1.) to two right angles;
Atherefore the two angles
E CEA, AED are equal to the two AED, DEB. Take away the common angle AED,
D and the remaining angle CEA is equal (3. Ax.) to the remaining angle DEB. In the same manner it may be demonstrated that the angles CEB, AED are equal. Therefore, if two straight lines, &c. Q. E. D.
Cor. 1. From this it is manifest, that if two straight lines cut one anotber, the angles which they make at the point of their intersection, are together equal to four right angles.
Cor. 2. And bence, all the angles made by any number of straight lines meeting in one point, are together equal to four right angles.
PROP. XVI. THEOR.
If one side of a triangle be produced, the exterior angle is greater than
either of the interior, and opposite angles.
Let ABC be a triangle, and let its side BC be produced to D, the exterior angle ACD is greater than either of the interior opposite angles CBA, BAC. Bisect (10. 1.) AC in E, join
E BE and produce it to F, and make EF equal to BE; join also FC, and produce AC to G.
Because AE is equal to EC, and BE to EF ; AE, EB are equal to CE, EF, each to each ; and the angle AEB is equal(15. 1.) to the angle CEF, because they are vertical angles ; therefore the base AB is equal (4. 1.)
B to the base CF, and the triangle AEB to the triangle CEF, and the remaining angles to the remaining angles, each to each, to which the equal sides are oppo
G site; wherefore the angle BAE is equal to the angle ECF; but the angle ECD is greater than the angle ECF ; therefore the angle ECD, that is ACD, is greater than BAĚ: In the same manner, if the side
BC be bisected, it may be demonstrated that the angle BCG, that is (15. 1.), the angle ACD, is greater than the angle ABC. Therefore if one side, &c. Q. E. D.
PROP. XVII. THEOR.
Any two angles of a triangle are together less than two right angles.
Let ABC be any triangle; any
A two of its angles together are less than two right angles.
Produce BC to D; and because ACD is the exterior angle of the triangle ABC, ACD is greater (16. 1.) then the interior and opposite angle ABC ; to each of these add the angle ACB; therefore the apgles ACD, ACB are greater than the angles ABC,
B ACB; but ACD, ACB are to
D gether equal (13. 1) to two right angles; therefore the angles ABC, BCA are less than two right angles. In like manner, it way be demonstrated, that BAC, ACB, as also, CAB, ABC, are less than two right angles. Therefore, any two angles, &c. Q. E. D.
PROP. XVIII. THEOR.
The greater side of every triangle has the greater angle opposite to it.
Let ABC be a triangle of which
A the side AC is greater than the side AB; the angle ABC is also greater than the angle BCA.
D From AC, which is greater than AB, cut off (3. 1.) AD equal to AB and join BD ; and because ADB is the exterior angle of the triangle
B BDC, it is greater (16. 1.) than the interior and opposite angle DCB; but ADB is equal (5. 1.) to ABD because the side AB is equal to the side AD; therefore the angle ABD is likewise greater than the angle ACB ; wherefore much more is the angle ABC greater than ACB. Therefore the greater side, &c. Q. E. D.
PROP. XIX. THEOR.
The greater angle of every triangle is subtended by 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 side AC is likewise greater than the side AB. For, if it be not greater, AC must
A either be equal to AB, or less than it ; it is not equal, because then the angle ABC would be equal(ö. 1.) to the angle ACB ; but it is not; therefore AC is not equal to AB ; neither is it
3; less because then the angle ABC would be less (18. 1.) than the angle
B ACB; but it is not; therefore the side AC is not less than AB; and it has been shewn that it is not equal to AB ; therefore AC is greater than AB. Wherefore the greater angle, &c. Q. E. D.
PROP. XX. THEOR.
Any 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 side, 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,
D and make (3. 1.) AD equal to AC ; and join DC.
Because DA is equal to AC, the angle ADC is likewise equal (5. 1.) to ACD ; but the angle BCD is greater than the angle ACD ; therefore the angle BCD
B is greater than the angle ADC; and because the angle BCD of the triangle DCB is greater than its angle BDC, and that the greater (19. 1.) side is opposite to the greater angle : therefore the side DB is greater than the side BC ; but DB is equal to BA and AC together, therefore BA and AC together are greater than BC. Io the same manner it may be demonstrated, that the sides AB, BC are greater than CA, and BC, CA greater than AB. Therefore any two sides, &c. Q. E. D.