PROPOSITION I. PROBLEM.

To describe an equilateral triangle upon a given finite straight line.

Let AB be the given straight line; it is required to describe an equilateral triangle upon it.

From the centre A, at the dis

C

tulate.

* 3 Pos- tance AB, describe* the circle BCD, and from the centre B, at the distance BA, describe the circle ACE; and from the point C, in which the circles cut one

D

A

B

* 1 Post. another, draw the straight lines* CA, CB, to the points A, B; ABC shall be an equilateral triangle.

finition.

Because the point A is the centre of the circle BCD, * 15 De- AC is equal* to AB; and because the point B is the centre of the circle ACE, BC is equal to BA: but it has been proved that CA is equal to AB; therefore CA, CB, are each of them equal to AB; but things * 1st which are equal to the same thing are equal* to one Axiom. another; therefore CA is equal to CB; wherefore CA, AB, BC, are equal to one another; and the triangle ABC is therefore equilateral, and it is described upon the given straight line AB. Which was required to be done.

From a given point to draw a straight line equal to a given straight line.

Let A be the given point, and BC the given straight line; it is required to draw from the point A a straight line equal to BC.

From the point A to B draw the straight line AB; and upon it describe the equilateral triangle DAB, * 2 Post. and produce the straight line DB to E; from the * 3 Post. centre B, at the distance BC, describe the circle CFH, and from the centre D, at the distance DF, describe the circle FGK: produce the straight line DA to meet the circle FGK in L; AL shall be equal to BC.

shown, that BC is equal to BF; wherefore AL and BC are each of them equal to BF; and things that are equal to the same thing are equal to one another; * 1 Ax,

therefore the straight line AL is equal to BC. Wherefore, from the given point A, a straight line AL has been drawn equal to the given straight line BC. Which was to be done.

PROP. III. PROB.

From the greater of two given straight lines to cut off a part equal to the less.

Let AB and C be the two given straight lines, whereof AB is the greater. It is required to cut off from AB, the greater, a part equal to C, the less.

From the point A draw the

D

A

E

B

F

* 2. 1.

straight line AD equal to C; and from the centre A, and at the distance AD, describe the circle DEF: *3 Post. then AE shall be equal to C. Because A is the centre of the circle DEF, AE is equal to AD; but the * 15 Def. straight line C is likewise equal to AD; whence AE

and C are each of them equal to AD; wherefore the straight line AE is equal to C, and therefore from * 1 Ax.

AB, the greater of two straight lines, a part AE has

been cut off equal to C the less. Which was to be done.

PROP. IV. THEOREM.

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; they shall likewise have their bases, or third sides, equal; and the two triangles shall be equal; and their other angles shall be equal, each to each, viz. those to which the equal sides are opposite.

Let ABC, DEF be two tri- A

D

B

E

F

angles, which have the two
sides AB, AC equal to the
two sides DE, DF, each to
each, viz. AB to DE, and AC
to 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 triangle
DEF; and the other angles to which the equal sides
are opposite, shall be equal, each to each, viz. the
angle ABC to the angle DEF, and the angle ACB to
the angle 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 was proved to coincide with the point E; wherefore the base BC shall coincide with the base EF; because the point B coinciding with E, and C with F, if the base BC did not coincide with the base EF, two straight * 10 x. lines would enclose a space, which is impossible.* Therefore the base BC coincides with the base EF, and is therefore equal to it. Wherefore the whole triangle ABC coincides with the whole triangle DEF, and is equal to it; and the other angles of the one coincide with the remaining angles of the other, and are 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 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 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 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 greater, cut off AG equal to AF, the less, and join FC, GB. * 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

* 4. 1.

ACF to the angle ABG, and the angle AFC to the angle AGB: and because the whole AF is equal to the whole AG, of which the parts AB, AC, are equal; the * 3 Ax. remainder BF is equal 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; and the angle BFC has been proved to be equal to the angle CGB; wherefore the two triangles BFC, CGB are equal, and their remaining angles are equal, 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 triangle be equal to one another, the sides also which subtend, or are opposite to, the equal angles, shall be equal to one another.

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

For, if AB be not equal to AC, one of them is greater 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