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Book I. AB to DE, and AC to

D
DF; and the angle BAC
equal to the angle EDF;
the base BC shall be equal
to the base EF; and the
triangle ABC to the tri-
angle DEF; and the other
angles, to which the equal
sides are opposite, shall be B

CE F
equal each to each, viz. the
angle ABC to the angle DEF, and the angle ACB to DFE.

For, if the triangle ABC be applied to DEF; so that the point A may be on D, and the straight line AB upon DE; the point B shall coincide with the point E, because AB is equal to DE; and AB coinciding with DE, AC shall coincide with DF, because the angle BAC is equal to the angle EDF; wherefore also the point C shall coincide with the point F, because the straight line AC is equal to DF; but the point B coincides with the point E; wherefore the base BC shall coincide with the base EF, because the point B coi ciding with E, and C with F, if the base BC does not coincide with the

base EF, two straight lines would enclose a space, which is a 10. Ax. impossible a. Therefore the base BC shall coincide with the

base EF, and be equal to it. Wherefore the whole triangle ABC shall coincide with the whole triangle DEF, and be equal to it; and the other angles of the one shall coincide with the remaining angles of the other, and be equal to them, viz. the angle ABC to the angle DEF, and the angle ACB to DFE. Therefore, “ if two triangles have two sides of the one equal to two sides of the other, each to each, and have likewise the angles contained by those sides equal to one another, their bases shall likewise be equal, and the triangles be equal, and their other angles to which the equal sides are opposite shall be equal, each to each." Which was to be demonstrated.

PROP. V. THEOR.

The angles at the base of an Isosceles triangle are equal to one another ; and if the equal sides be produced, the angles upon the other side of the base shall be equal.

Let ABC be an Isosceles triangle, of which the side AB is

equal to AC, and let the straight line AB, AC be produced Book I. to D and E; the angle ABC shall be equal to the angle ACB, and the angle CBD to the angle BCE.

In BD take any point F, and from A E the greater, cut off AG equal a to AF the less, and join FC, GB.

a 3. 1. Because AF is equal to AG, and AB to AC, the two sides FA, AC are equal to the two GA, AB, each to each; and they contain the angle FAG common to the two triangles AFC,

A AGB; therefore the base FC is equal to the base GB, and the tri

b 4. 1. angle AFC to the triangle AGB; and the remaining angles of the one are equal to the remaining angles of the other, each to each, to which

B

C the equal sides are opposite; viz. the angle ACF to the angle ABG, F

G and the angle AFC to the angle AGB: And because the whole AF is equal to the whole AG, of which D

E the parts AB, AC, are equal; the remainder BF shall be equal to the remainder CG; and FC c. 3. Ax. was proved to be equal to GB; therefore the two sides, BF, FC are equal to the two CG, GB, each to each ; and the angle BFC is equal to the angle CGB, and the base BC is common to the two triangles BFC, CGB; wherefore these triangles are equal b, and

their remaining angles, each to each, to which the equal sides are opposite; therefore the angle FBC is equal to the angle GCB, and the angle BCF to the angle CBG: And, since it has been demonstrated, that the whole angle ABG is equal to the whole ACF, the parts of which, the angles CBG, BCF are also equal; the remaining angle ABC is therefore equal to the remaining angle ACB, which are the angles at the base of the triangle ABC: And it has also been proved that the angle FBC is equal to the angle GCB, which are the angles upon the other side of the base. Therefore, “ the angles at the base," &c. Q. E. D.

Corollary. Hence every equilateral triangle is also equiangular.

PROP. VI. THEOR.

If two angles of a iriangle be equal to one another, the sides also which subtend, or are opposite to, the equal angles, shall be equal to one another.

Book I.

Let ABC be a triangle having the angle ABC equal to the angle ACB; the side AB is also equal to the side AC.

For, if AB be not equal to AC, one of them is greater than a 3. ). the other: Let AB be the greater, and from it cut a off DB equal to AC, the less, and join DC;

A
therefore, because in the triangles
DBC, ACB, DB is equal to AC, and
BC common to both, the two sides DB,

D
BC are equal to the two AC, CB, each
to each; and the angle DBC is equal
to the angle ACB; therefore the base

DC is equal to the base AB, and the
b 4. 1. triangle DBC is equal to the triangle
ACB, the less to the greater; which B

C is absurd. Therefore AB is not unequal to AC, that is, it is equal to it. Wherefore, if two angles," &c. Q. E. D.

Cor. Hence every equiangular triangle is also equilateral.

1

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PROP. VII. THEOR.

See N.

Upon the same base, and on the same side of it,
there cannot be two triangles that have their sides
which are terminated in one extremity of the base
equal to one another, and likewise those which are
terminated in the other extremity.

If it be possible, let there be two triangles ACB, ADB, up-
on the same base AB, and upon the same side of it, which have
their sides CA, DA, terminated in the extremity A of the base
equal to one another, and likewise
their sides CB, DB, that are termi-

С
nated in B.

D
Join CD; then in the case in
which the vertex of each of the tri-
angles is without the other triangle,
because AC is equal to AD, the
angle ACD is equal a to the angle
ADC: But the angle ACD is great-
er than the angle BCD; therefore
the angle ADC is greater also than A

B
BCD; much more then is the angle
BDC greater than the angle BCD. Again, because CB is
equal to DB, the angle BDC is equala to the angle BCD; but it
has been demonstrated to be greater than it; which is impossible.

a 5. 1.

D

But if one of the vertices, as D, be within the other triangle Book I. ACB; produce AC, AD to E, F; therefore, because AC is equal to AD in the

E triangle ACD, the angles ECD, FDC

F upon the other side of the base CD are equal a to one another, but the

a 5. 1. angle ECD is greater than the angle BČD; wherefore the angle FDC is likewise greater than BCD; much more then is the angle BDC greater than the angle BCD. Again, because CB is equal to DB, the angle BDCA

B is equal a to the angle BCD; but BDC has been proved to be greater than the same BCD; which is impossible. The case in which the vertex of one triangle is upon a side of the other, needs no demonstration. Therefore 66

upon the same base, and on the same side of it, there cannot be two triangles that have their sides which are terminated in one extremity of the base equal to one another, and likewise those which are terminated in the other extremity.Q. E. D.

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 contain: ed by the two sides equal to them of the other.

Let ABC, DEF be two triangles, having the two sides AB,
AC, equal to the two sides DE, DF, each to each, viz. AB to
DE, and AC to
DF; and also the A

D G
base BC equal to
the base EF. The
angle BAC is e-
qual to the angle
EDF.

For, if the triangle ABC be applied to DEF, so B CE

F that the point B be on E, and the straight line BC upon EF; the point C shall

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Book I. 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 the base BC coincides with the base
EF, but the sides BA, CA do not coincide with the sides ED,
FD, but have a different situation as EG, FG: then, upon
the same base EF, and upon the same side of it, there can be
two triangles that have their sides which are terminated in one
extremity of the base equal to one another, and likewise their

sides terminated in the other extremity: But this is imposa 7. 1.

sible a ; therefore, if the base BC 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 b 8. Ax. EDF, and is equalo to it. Therefore if two triangles,&c.

Q. E. D.

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a 3. 1.

bl. I.

To biscct a given rectilineal angle, that is, to divide it into two equal angles.

Let BAC be the given rectilineal angle, it is required to bisect it.

Take any point D in AB, and from AC cut a off AE equal
to AD; join DE, and upon it de-

А
scribe an equilateral triangle DEF;
then join AF; the straight line AF
bisects the angle BAC.
Because AD is equal to AE, and

D

E
AF is common to the two triangles
DAF, EAF; the two sides DA,
AF, are equal to the two sides EA,
AF, each to each; and the base DF

B

F
is equal to the base EF; therefore
the angle DAF is equal o to the an-
gle EAF; wherefore the given rectilineal angle BAC is bi-
sected by the straight line AF. Which was to be done.

PROP. X. PROB.
To bisect a given finite straight line, that is, to di-
vide it into two equal parts.

Let AB be the given straight line; it is required to divide
it into two equal parts.

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 8. 1.

a l. 1. b 9. 1.

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