c

Book I. these the angle BGH, therefore the angles EGB, BGH are equal to the angles BGH, GHD; but EGB, BGH are equal to two right angles; therefore also BGH, GHD are equal to two right angles. wherefore if a straight line, &c. Q. E. D.

C. 13. I.

a. 29. I.

b. 27. I.

TRAIGHT lines which are parallel to the fame ftraight line, are parallel to one another.

S S

Let AB, CD be each of them parallel to EF; AB is alfo parallel to CD.

Let the ftraight line GHK cut AB, EF, CD; and because GHK cuts the parallel ftraight lines AB,

a

EF, the angle AGH is equal to
the angle GHF. again, because the
ftraight line GK cuts the parallel A
ftraight lines EF, CD, the angle
GHF is equal a to the angle E
GKD. and it was shewn that the C

angle AGK is equal to the angle
GHF; therefore also AGK is e-
qual to GKD. and they are alter-

nate angles; therefore AB is parallel to CD. wherefore straight lines, &c. Q. E. D.

10 draw a straight line thro' a given point parallel to a given straight line.

Let A be the given point, and BC the given straight line; it is

required to draw a ftraight line thro'.

the point A, parallel to the ftraight E

line BC.

In BC take any point D, and join

AD; and at the point A in the straight

a. 23. I.

line AD make the angle DAE e-B

D

C

qual to the angle ADC; and produce the straight line EA to F. Because the ftraight line AD which meets the two straight lines BC, EF, makes the alternate angles EAD, ADC equal to one b. 27. 1. another, EF is parallel b to BC. therefore the ftraight line EAF is

drawn thro' the given point A parallel to the given ftraight line Book I. BC. Which was to be done.

F a fide of any triangle be produced, the exterior angle is equal to the two interior and oppofite angles; and the three interior angles of every triangle are equal to two right angles.

Let ABC be a triangle, and let one of its fides BC be produced to D. the exterior angle ACD is equal to the two interior and oppofite angles CAB, ABC; and the three interior angles of the triangle, viz. ABC, BCA, CAB are together equal to two

right angles.

Thro' the point C draw CE parallel to the straight line AB. a. st. 1.

and because AB is parallel to

and opposite angle ABC. butB

the angle ACE was fhewn to be equal to the angle BAC; therefore the whole exterior angle ACD is equal to the two interior and oppofite angles CAB, ABC. to thefe equals add the angle ACB, and the angles ACD, ACB are equal to the three angles CBA, BAC, ACB. but the angles ACD, ACB are equal to two right angles; c. 13. 1. therefore also the angles CBA, BAC, ACB are equal to two right angles. wherefore if a fide of a triangle, &c. Q. E. D.

Book I. angles. And, by the preceding Propofition, all the angles of these

triangles are equal to twice as many right angles as there are tri

angles, that is, as there are fides of the figure. and the same angles are equal to the angles of the figure, together with the angles at a. 2. Cor. the point F which is the common Vertex of the triangles; that is a, together with four right angles. Therefore all the angles of the figure, together with four right angles, are equal to twice as many right angles as the figure has fides.

15. I.

b. 13. I.

COR. 2. All the exterior angles of any rectilineal figure are together equal to four right angles.

Becaufe every interior angle
ABC with its adjacent exterior
ABD is equal to two right an-
gles; therefore all the interior to-
gether with all the exterior angles
of the figure, are equal to twice
as many right angles as there are
fides of the figure, that is, by the
foregoing Corollary, they are equal
D

A

B

to all the interior angles of the figure, together with four right angles. therefore all the exterior angles are equal to four right angles.

THE

HE ftraight lines which join the extremities of two equal and parallel ftraight lines, towards the

fame parts, are also themselves equal and parallel.

Let AB, CD be equal and parallel straight lines, and joined to wards the fame parts by the ftraight A

B

lines AC, BD; AC, BD are alfo

A. 29. 1.

b. 4. I.

are equal; and because AB is equal to CD, and BC common to the two triangles ABC, DCB, the two fides AB, BC are equal to the two DC, CB; and the angle ABC is equal to the angle BCD; therefore the base AC is equal to the bafe BD, and the triangle ABC to the triangle BCD, and the other angles to the other an

b. 4. I.

gles, each to each, to which the equal fides are oppofite. there- Book I. fore the angle ACB is equal to the angle CBD. and because the straight line BC meets the two straight lines AC, BD and makes, the alternate angles ACB, CBD equal to one another, AC is rallel to BD. and it was fhewn to be equal to it. therefore c. 27. I. ftraight lines, &c. Q. E, D. Gry

TH

PROP. XXXIV. THEOR.

HE oppofite fides and angles of parallelograms are equal to one another, and the diameter bifects them, that is, divides them into two equal parts.

N. B. A Parallelogram is a four fided figure of which the oppofite fides are parallel. and the diameter is the Atraight line joining two of its oppofite angles.

Let ABCD be a parallelogram, of which BC is a diameter. the oppofite fides and angles of the figure are equal to one another; and the diameter BC bifects it.

A

B

Because AB is parallel to CD, and BC meets them, the alternate angles ABC, BCD are equal to one another, and because AC is parallel to BD, and BC meets them, the alternate angles ACB, CBD are equal a to one another. wherefore the two triangles ABC, CBD have two angles ABC, BCA

a 29. I.

in one, equal to two angles BCD, CBD in the other, each to each, and one fide BC common to the two triangles, which is adjacent to their equal angles; therefore their other fides fhall be equal, each to each, and the third angle of the one to the third angle of the other, viz. the fide AB to the fide CD, and AC to BD, and b. 26. 1. the angle BAC equal to the angle BDC. and because the angle ABC is equal to the angle BCD, and the angle CBD to the angle ACB; the whole angle ABD is equal to the whole angle ACD. and the angle BAC has been fhewn to be equal to the angle BDC; therefore the oppofite fides and angles of parallelograms are equal to one another. alfo, their diameter bifects them. for, AB being equal to CD, and BC common; the two AB, BC are equal to the two DC, CB, each to each; and the angle ABC is equal to the

c

Book I. angle BCD; therefore the triangle ABC is equal to the triangle BCD, and the diameter BC divides the parallelogram ACDB into two equal parts. Q. E. D.

C. 4. I.

See N.

See the

2d and 3d. Figures.

a. 34. I.

PA

ARALLELOGRAMS upon the fame bafe and between the fame parallels, are equal to one another.

Let the parallelograms ABCD, EBCF be upon the fame base BC and between the fame parallels AF, BC. the parallelogram ABCD fhall be equal to the parallelogram EBCF.

If the fides AD, DF of the parallelograms ABCD, DBCF oppofite to the base BC, be terminated in the fame point D; it is plain that each of the parallelograms is double of thetriangle BDC; and they are therefore equal to one another.

A

But if the fides AD, EF oppofite to the bafe BC of the parallelograms ABCD,, EBCF be not terminated in the fameB

D

F

a

point; then because ABCD is a parallelogram, AD is equal to BC; for the fame reason, EF is equal to BC; wherefore AD is h. 1, Ax. equal to EF; and DE is common; therefore the whole, or the

C

c 2. or 3. remainder, AE is equal to the whole, or the remainder DF; AB alfo is equal to DC; and the two EA, AB are therefore equal

Ax.

A

DE

FAED F

d. 29. I. e. 4. I.

e

to the two FD, DC, each to each; and the exterior angle FDC is equal to the interior EAB; therefore the base EB is equal to the base FC, and the triangle EAB equal to the triangle FDC. take the triangle FDC from the trapezium ABCF, and from the fame trapezium take the triangle EAB; the remainders therefore are £. 3. Ax. equal, that is, the parallelogram ABCD is equal to the parallelogram EBCF. therefore parallelograms upon the fame base, &c. Q. E. D,