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angle DEF, so that the spaces which they contain or their Book I. areas are equal: and the remaining 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 the angle 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 be equal, and their areas shall 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 N are equal to one another; and if the equal sides be produced, the angles upon the other side of the base shall also be equal.

Let ABC be an isosceles triangle, of which the side AB is equal to AC, and let the straight lines AB, AC be produced 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 AE the greate cut off AG equal a to AF the less, and join FC, GB. 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

b

B

a 3. 1.

b 4, 1.

C

G

E

angle FAG common to the
two triangles, AFC, AGB;
therefore the base FC is
equal to the base GB, and
the triangle AFC to the
triangle AGB; and the re-
maining angles of the one
are equal to the remain-
ing angles of the other, each
to each, to which the equal
sides are opposite, viz. the
angle ACF to the angle
ABG, and the angle AFC to the angle AGB: And be-
cause the whole AF is equal to the whole AG, and the
part AB to the part AC; the remainder BF is e-

D

F

a 3 Ax.

a

Book I. qual to the remainder CG; and FC was proved to be equal to GB, therefore the two sides BF, FC are equal to the two CG, GB, each to each; but the angle BFC is equal to the angle CGB; wherefore the triangles BFC, CGB are equal b, and their remaining angles are equal, 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. Now, since it has been demonstrated, that the whole angle ABG is equal to the whole ACF, and the part CBG to the part BCF, 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 triangle be equal to one another, the sides which subtend, or are opposite to them, are also equal to one another.

D

b

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 b3 1. than the other: Let AB be the greater, and from it cut off DB equal to AC the less, and join DC; therefore, because in the triangles DBC, ACB, DB is equal to AC, and BC common to both, the two sides DB, BC are equal to the two AC, CB, each to each; but the angle DBC is also equal to the angle ACB; therefore the base DC is equal to the base AB, and the area of the triangle DBC is equal to that c. 4. 1. of the triangle ACB, the less to the greater; which is absurd. There

B

fore, 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.

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PROP. VII. THEOR.

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, equal to one another.

Let there be two triangles ACB, ADB, upon the same
base AB, and upon the same side of it, which have their
sides CA, DA, terminated in A equal to one another;
then their sides CB, DB,
terminated in B, cannot be
equal to one another.

. Join CD, and if possible
let CB be equal to DB; then,
in the case in which the ver-
tex of each of the triangles is
without the other triangle,
because AC is equal to AD,
the angle ACD is equal a to
the angle ADC: But the A

C

D

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

a

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

ECD, FDC upon the

other side of the base

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CD are equal a to one another, but the angle ECD is greater than the angle BCD; wherefore the angle FDC is likewise greater than BCD; much more then is the

Book I.

N.

a 5. 1.

3

Book I. angle BDC greater than the angle BCD. Again, because CB is equal to DB, the angle BDC is equal 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, 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 equal to one another. 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 contained by the two sides 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 base BC

D G

A A

B

E

F

equal to the 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, Book I. 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 another, and likewise their sides terminated in the other extremity; but this is impossible; there a 7. 1. fore, if the base BC coincide 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 it. Therefore if two triangles, &c. Q. E. D.

PROP. IX. PROB.

To bisect 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.

b

A

a

b8. Ax.

24

b. 1. 1.

Take any point D in AB, and from AC cut off AE a 3. 1. equal to AD; join DE, and upon it describe an equilateral triangle 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, 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 angle B EAF: wherefore the given rec

c

D

E

F

tilineal angle BAC is bisected by 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.

c 8. I

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