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Book I. been proved to be greater than the same BCD; which is im-,
possible. The case in which the vertex of one triangle is up-
Therefore, upon the same base, and on the same side of it,
PROP. VIII. THEOR.
IF two triangles have two sides of the one equal to two sides of the other, each to each, and have likewise their bases equal; the angle which is contained .. by the two sides of the one shall be equal to the angle contained by the two sides of the other.
Let ABC, DEF be two triangles having the two sides AB,
base EF: the angle BAC is equal to the angle EDF.
For, if the triangle ABC be applied to the triangle DEF, so that the point B be on E, and the straight line BC upon EF; the point C shall also coincide with the point F, because BC is equal to EF: therefore BC coinciding with EF, BA and AC shall coincide with ED and DF; for, if BA and CA do not coincide with ED and FD, but have a different situation, as EG and FG; then, upon the same base EF, and upon the same side of it, there can be two triangles EDF, EGF, that have their sides which
are terminated in one extremity of the base equal to one Book I. another, and likewise their sides terminated in the other extremity: but this is impossiblea ; therefore, if the base BC a 7. 1. coincides with the base EF, the sides BA, AC cannot but coincide with the sides ED, DF; wherefore likewise the angle BAC coincides with the angle EDF, and is equal to b 8. Ax. it. Therefore, if two triangles, &c. Q. E. D.
PROP. IX. PROB.
TO bisect a given rectilineal angle, that is, to di. vide it into two equal angles.
Let BAC be the given rectilineal angle; it is required to bisect it.
Take any point Din AB, and from AC cuta off AE a 3. 1. equal to AD; join DE, and upon it describe b an equilateral triangle
b 1. 1. DEF; then join AF; the straight line AF bisects the angle BAC.
Because AD is equal to AE, and AF is common to the two triangles · DAF, EAF, the two sides DA,
E AF are equal to the two sides EA, AF, each to each; but the base DF is also equal to the base EF; therefore the angle DAF is equal to the
c 8. 1. angle EAF; wherefore the given rectilineal angle BAC is bisected by B the straight line AF. Which was to be done.
PROP. X. PROB.
TO bisect a given finite straight line, that is, to divide it into two equal parts.
Let AB be the given straight line; it is required to divide it into two equal parts.
Describea upon it an equilateral triangle ABC, and bi- a 1.1. sectb the angle ACB by the straight line CD. AB is cut b 9. 1. into two equal parts in the point D.
TO draw a straight line at right angles to a given straight line, from a given point in the same.
a 3. 1. b 1.1.
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.
Take any point D in AC, and makea CE equal to CD, and upon DÊ describeb the
E B В 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; therefore each of the angles DCF, ECF is a rights 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.
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.
E Take any point D upon the other side of AB,
A F H and from the centre C, at
B the distance CD describea
a 3. Post. the circle EGF meeting AB in F, G, and bisectb FG in b 10. 1 H, and 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; now the base CF is also equal“ c 11. Def, to the base CG; therefore the angle CHF is equals to the d 8. 1. angle CHG; and they are adjacent angles; but 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 per- e 7. Def. pendicular 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
B a Def. 7. a right anglea ; but, if not, from the point B draw BE at b 11. 1. right anglesb 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 c 2: Ax. equal to the three CBA, ABE, EBD. Again, the angle
DBA is equal to the two angles DBE, EBĂ; 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; therefore the angles CBE, EBD are d 1. Ax. equald to the angles DBA, ABC; but ČBE, 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 ad. jacent 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