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Book I,

a 1. 1. b 9. 1.

Describe a upon it an equilateral triangle ABC, and bisect the angle ACB by the straight line CD. AB is cut into two equal parts in the point D.

C

Because AC is equal to CB, and CD common to the two triangles ACD, BCD; the two sides AC, CD, are equal to the two BC, CD, each to each; but the angle ACD is also equal to the angle BCD; therefore the base AD is e 4. 1. equal to the base DB, and the straight line AB is divided into two equal parts in the point D. Which was to be done.

a 3. 1.

b l. I.

c 8. 1.

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A

D

B

PROP. XI. PROB.

To draw a straight line at right angles to a given straight line, from a given point in that line.

Let AB be a given straight line, and C a point given in it; it is required to draw a straight line from the point C at right angles to AB.

F.

Take any point D in AC, and a make CE equal to CD, and upon DE describe the equilateral triangle DFE, and join FC; the straight line FC, drawn from the given point C, is at right angles to the given straight line AB.

A D

C

E B

Because DC is equal to CE, and FC common to the two triangles DCF, ECF, the two sides DC, CF are equal to the two EC, CF, each to each; but the base DF is also equal to the base EF; therefore the angle DCF is equal to the angle ECF; and they are adjacent angles. But, when the adjacent angles which one straight line makes with another straight line are equal to one another, each of them is d 7. def. called a right angle; therefore each of the angles DCF, ECF, is a right angle. Wherefore, from the given point C, in the given straight line AB, FC has been drawn at right angles to AB. Which was to be done.

d

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 perpendicular to AB from the point C

Take any point D upon the other side of

AB, and from the cen

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Book I.

tre C, at the distance

b

CD, describe the circle EGF meeting AB in F, G; and a 3. Post. bisect ↳ FG in H, and join CF, CH, CG; the straight b 10. 1. line CH, drawn from the given point C, is perpendicular to the given straight line AB.

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1.

d 8. 1.

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 to the base CG; therefore the angle c 11. Def. CHF is equal 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.

C

Book I

a Def. 7.

For, if the angle CBA be equal to ABD, each of them is a right angle ; but, if not, from the point B draw BE

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b 11. 1.

2. Ax.

d1. Ax.

c

b

at right angles 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 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 angle 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 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.

d

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.

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a

For, if BD be not in the same straight line with CB, Book I. let BE be in the same straight line with it; 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 to two right angles; but the angles ABC, a 13. 1. 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 remaining angle ABE is equal to the remaining b 3. Ax. 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 may 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.

b

PROP. XV. THEOR.

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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 to two a 13. 1. right angles: and the C

angles AED, DEB which the straight line DE makes with the straight line AB arealsotogether equal a to two right angles; therefore the two angles CEA, AED are

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b

equal to the two AED, DEB. Take away the common angle AED, and the remaining angle CEA is equal ↳ b 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 another, the angles which they make at the point of their intersection, are together equal to four right angles.

Book I.

10. 1.

COR. 2. And hence, 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 oppo

site angles CBA, BAC.
Bisecta AC in E, join
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 anb 15. 1. gle AEB is equal to the angle CEF, because they are vertical angles; therefore the base AB is equal

c 4. 1.

c

b

B

G

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D

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 opposite; 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 BAE: In the same manner, if the side BC be bisected, it may be demonstrated that the angle BCG, that is ", the angle ACD, is greater than the angle ABC. Therefore, if one side, &c. Q. E. D.

PROP. XVII. THEOR.

Any treo angles of a triangle are together less than two right angles.

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