Because the point F is the centre of the circle DKL, FD is Book I. equal to FK; but FD is equal to the straight line A; therefore FK is equal to A: Again, because G is the centre of the c 15. Def. circle LKH, GH is equal to GK; but GH is equal to C; therefore also GK is equal to C; and FG is equal to B; therefore the three straight lines KF, FG, GK, are equal to the three A, B, C: And therefore the triangle KFG has its three sides KF, FG, GK equal to the three given straight lines, A, B, C. Which was to be done.

PROP. XXIII. PROB.

At a given point in a given straight line, to make a rectilineal angle equal to a given rectilineal angle.

Let AB be the given straight line, and A the given point in it, and DCE the given rectilineal angle, it is required to

make an angle at the

given point A in the

A

given straight line AB,

that shall be equal to

equal to the three

straight lines CD, DE, EC, so that CD be equal to AF, CE to AG, and DE to FG; and because DC, CE are equal to FA, AG, each to each, and the base DE to the base FG; the angle DCE is equal to the angle FAG. Therefore at 68. 1. the given point A in the given straight line AB, the angle FAG is made equal to the given rectilineal angle DCE. Which was to be done.

PROP. XXIV. THEOR.

If two triangles have two sides of the one equal See N. to two sides of the other, each to each, but the angle contained by the two sides of one of them greater than the angle contained by the two sides equal to them, of the other; the base of that which has the greater angle shall be greater than the base of the other.

Book I.

Let ABC, DEF be two triangles which have the two sides AB, AC equal to the two DE, DF, each to each, viz. AB equal to DE, and AC to DF; but the angle BAC greater than the angle EDF; the base BC is also greater than the base EF.

Of the two sides DE, DF, let DE be the side which is not greater than the other, and at the point D, in the straight line a 23. 1. DE, make a the angle EDG equal to the angle BAC; and make DG equal to AC or DF, and join EG, GF.

b 3. 1.

Because AB is equal to DE, and AC to DG, the two sides BA, AC are equal to the two ED, DG, each to each, and the angle BAC is equal

therefore the base

D

to the angle EDG; A

the angle DFG is greater than EGF; and much more is the angle EFG greater than the angle EGF; and because the angle EFG of the triangle EFG is greater than its angle EGF, e 19. 1. and because the greatere side is opposite to the greater angle; that side EG is therefore greater than the side EF; but EG is equal to BC; and therefore also BC is greater than EF. Therefore, " if two triangles," &c. Q. E. D.

PROP. XXV. THEOR.

If two triangles have two sides of the one equal to two sides of the other, each to each, but the base of the one greater than the base of the other; the angle also contained by the sides of that which has the greater base, shall be greater than the angle contained by the sides equal to them of the other.

Let ABC, DEF be two triangles which have the two sides AB, AC equal to the two sides DE, DF, each to each, viz. AB equal to DE, and AC to DF; but let the base CB be greater than the base EF; the angle BAC is likewise greater than the angle EDF.

For, if it be not greater, it must either be equal to it, or less; Book I. but the angle BAC is not equal to the angle EDF, because then

the base BC would

be equal to EF; but

it is not; therefore

the angle BAC is not A
equal to the angle
EDF; neither is it
less; because then

D

a 4. 1.

the angle EDF; and it was shown that it is not equal to it;

therefore the angle BAC is greater than the angle EDF.

Wherefore, " if two triangles," &c. Q. E. D.

PROP. XXVI. THEOR.

If two triangles have two angles of one equal to two angles of the other, each to each; and one side equal to one side, viz. either the sides adjacent to the equal angles, or the sides opposite to equal angles in each; then shall the other sides be equal, each to each; and also the third angle of the one to the third angle of

the other.

Let ABC, DEF be two triangles which have the angles ABC, BCA equal to the angles DEF, EFD, viz. ABC to DEF, and BCA to EFD, also one side equal to one side; and first let those sides be equal which are adjacent to the angles that are

equal in the two triangles; viz. BC to EF; the other sides

A

shall be equal, each G

to each, viz. AB to DE, and AC to DF; and the third angle BAC to the third angle EDF.

For, if AB be not B

equal to DE, one of them must be the greater. Let AB be the greater of the two, and make BG equal to DE, and join GC; therefore, because BG is equal to DE, and BC to EF, the two

a 4. 1.

a

Book I. sides GB, BC are equal to the two DE, EF, each to each; and the angle GBC is equal to the angle DEF; therefore the base GC is equal to the base DF, and the triangle GBC to the triangle DEF, and the other angles to the other angles, each to each, to which the equal sides are opposite; therefore the angle GCB is equal to the angle DFE; but DFE is, by the hypothesis, equal to the angle BCA; wherefore also the angle BCG is equal to the angle BCA, the less to the greater, which is impossible; therefore AB is not unequal to DE, that is, it is equal to it; and BC is equal to EF; therefore the two AB, BC are equal to the two DE, EF, each to each; but the angle ABC is equal to the angle DEF; therefore the base AC is equal to the base DF, and the third angle BAC to the third angle EDF.

For, if BC be not equal to EF, let BC be the greater of them, and make BH equal to EF, and join AH; and because BH is equal to EF, and AB to DE; the two AB, BH are equal to the two DE, EF, each to each; and they contain equal angles; therefore the base AH is equal to the base DF, and the triangle ABH to the triangle DEF, and the other angles are equal, each to each, to which the equal sides are opposite; therefore the angle BHA is equal to the angle EFD; but EFD is equal to the angle BCA; therefore also the angle BHA is equal to the angle BCA, that is, the exterior angle BHA of the triangle AHC is equal to its interior and opposite angle BCA; which is impossible; wherefore BC is not unequal to EF, that is, it is equal to it; and AB is equal to DE; therefore the two, AB, BC are equal to the two DE, EF, each to each, and they contain equal angles; wherefore the base AC is equal to the base DF, and the third angle BAC to the third angle EDF. Therefore, " if two triangles," &c. Q. E. D.

b

PROP. XXVII. THEOR.

If a straight line falling upon two other straight lines makes the alternate angles equal to one another, these two straight lines are parallel.

Book I.

Let the straight line EF, which falls upon the two straight lines AB, CD, make the alternate angles AEF, EFD equal to one another; AB is parallel to CD.

For, if it be not parallel, AB and CD being produced shall meet either towards B, D, or towards A, C; let them be produced and meet towards B, D in the point G; therefore GEF is a triangle, and its exterior angle AEF is greater than the a 16. 1.

interior and opposite angle

EFG; but it is also equal

to it, which is impossible; A

therefore AB and CD being

produced do not meet to

G

wards B, D. In like man

ner it may be demonstra- C/F

D

ted, that they do not meet

towards A, C; but those

straight lines which meet neither way, though produced ever

so far, are parallel to one another. AB therefore is parallel b 35. Def.

to CD. Wherefore, " if a straight line," &c. Q. E. D.

PROP. XXVIII. THEOR.

If a straight line falling upon two other straight lines makes the exterior angle equal to the interior and opposite upon the same side of the line ; or makes the interior angles upon the same side together equal to two right angles; the two straight lines are parallel to one another.

Because the angle EGB is equal to the angle GHD, and