PROPOSITION XIX. THEOREM.

The greater angle of every triangle is subtended by the greater side, or has the greater side opposite to it.

A

Let ABC be a triangle, of which the angle ABC is greater than the angle BCA; the side AC is likewise greater than the side AB.

For, if it be not greater, AC must either be equal to AB, or less than it; it is not

equal, because then the angle ABC would be equal to the angle ACB (I. 5); but it is not; therefore AC is not equal to AB; neither is it less; because then the angle ABC would be less than the angle ACB (I. 18); but it is not; therefore the side AC is not less than AB; and it has been shown that it is not equal to AB; therefore AC is greater than AB.

PROPOSITION XX. THEOREM.

Any two sides of a triangle are together greater than the third side.

Let ABC be a triangle; any two sides of it together are greater than the third side; namely, the sides BA, AC, greater than the side BC; and AB,

BC, greater than AC; and BC, CA, greater than AB.

Produce BA to the point D, and make (I. 3) AD equal to AC; and join DC.

B

Because DA is equal to AC, the angle ADC is likewise equal to ACD (I. 5); but the angle BCD is greater than the angle ACD; therefore the angle BCD is greater than the angle ADC; and because the angle BCD of the triangle DCB is greater than its angle BDC, and that the greater side is opposite to the greater angle (I. 19); therefore the side DB is greater than the side BC; but DB is equal to BA and AC; therefore the sides BA, AC, are greater than BC. In the same manner it may be demonstrated, that the sides AB, BC, are greater than CA, and BC, CA, greater than AB.

PROPOSITION XXI. THEOREM.

If from the ends of the side of a triangle, there be drawn two straight lines to a point within the triangle, these two lines shall be less than the other two sides of the triangle, but shall contain a greater angle.

Let the two straight lines BD, CD, be drawn from B, C, the ends of the side BC of the triangle ABC, to the point D within it; BD and DC are less than the other two sides BA, AC, of the triangle, but contain an angle BDC greater than the angle BAC.

A

Produce BD to E; and because two sides of a triangle are greater than the third side, the two sides BA, AE, of the triangle ABE are greater than BE. To each of these add EC; therefore the sides BA, AC, are greater than BE, EC. Again, because the two sides CE, ED, of the triangle CED are greater than CD, add DB to each of these; therefore the sides CE, E EB, are greater than CD, DB; but it has been shown that BA, AC, are greater than BE, EC; still greater, then, are BA, AC, than BD, DC.

Again, because the exterior angle of a triangle is greater than the interior and opposite angle, the exterior angle BDC of the triangle CDE is greater than CED; for the same reason, the exterior angle CEB of the triangle ABE is greater than BAC; and it has been demonstrated that the angle BDC is greater than the angle CEB; still greater, then, is the angle BDC than the angle BAC.

PROPOSITION XXII. PROBLEM.

To make a triangle of which the sides shall be equal to three given straight lines; but any two whatever of these lines must be greater than the third (I. 20).

Let A, B, C, be the three given straight lines, of which any two whatever are greater than the third; namely A and B greater than C, A and C greater than B, and B and C than A. It is required to make a triangle of which the sides shall be equal to A, B, C, each to each.

Take a straight line DE terminated at the point D, but

unlimited towards E, and make (I. 3) DF equal to A, FG to B, and GH equal to C; and from the centre F, with the radius FD, describe the circle DKL

(Post. 3); and from the centre G, with the radius GH, describe another circle D HLK, and join KF, KG; the triangle KFG has its sides equal to the three straight lines A, B, C.

Because the point F is the centre of the circle DKL, FD is equal to FK (Def. 14); but FD is equal to the straight line A; therefore FK is equal to A. Again, because G is the centre of the 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.

PROPOSITION XXIII. PROBLEM.

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 given straight line AB, that shall be equal to the given rectilineal angle DCE.

Take in CD, CE, any points D, E, and join DE; and make (1.22) the

A

triangle AFG, the sides of which shall be equal to the three straight lines CD, DE, CE, 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 (I. 8). Therefore, at the given point A in the given straight line AB, the angle FAG is made equal to the given rectilineal angle DCE.

PROPOSITION XXIV. THEOREM.

If two triangles have two sides of the one equal 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.

Let ABC, DEF, be two triangles which have the two sides AB, AC, equal to the two DE, DF, each to each; namely, 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 B

A

H

G

than the other, and at the point D, in the straight line DE, make (I. 23) the angle EDG equal to the angle BAC; and make DG equal to AC or DF (I. 3), and join EG, GF.

Because DE is not greater than DF or DG, therefore the angle DGE is not greater than DEG; but angle DHG is greater than DEG (I. 16); therefore DHG is greater than DGE, and hence the side DG or DF is greater than DH; therefore the line EG must lie between EF and ED, or the point F must be below EG.

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 to the angle EDG; therefore the base BC is equal to the base EG (I. 4); and because DG is equal to DF, the angle DFG is equal to the angle DGF (1.5); but the angle DGF is greater than the angle EGF; therefore the angle DFG is greater than EGF; and still greater is the angle EFG than the angle EGF; and because the angle EFG of the triangle EFG is greater than its angle EGF, and that the greater side is opposite to the greater angle (I. 19), the side EG is therefore greater than the side EF; but EG is equal to BC; and therefore also BC is greater than EF.

PROPOSITION XXV. THEOREM.

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 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; namely, AB equal to DE, and A

AC to DF; but the base CB is greater than the base EF; the angle BAC is likewise greater than the angle EDF.

B

For, if it be not greater, it must either be equal to it, or less; but the angle BAC is not equal to the angle EDF, because, then, the base BC would be equal to EF (I. 4), but it is not; therefore the angle BAC is not equal to the angle EDF; neither is it less; because then the base BC would be less than the base EF (I. 24); but it is not; therefore the angle BAC is not less than the angle EDF; and it was shown that it is not equal to it; therefore the angle BAC is greater than the angle EDF.

PROPOSITION XXVI. THEOREM.

If two triangles have two angles of the one equal to two angles of the other, each to each, and one side equal to one side; namely, either the sides adjacent to the equal angles, or the sides opposite to the 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.

Or, if two angles and a side in one triangle be respectively equal to two angles and a corresponding side in another triangle, the two triangles shall be equal in every respect.

D

Let ABC, DEF, be two triangles which have the angles ABC, BCA, equal to the angles DEF, EFD; namely, ABC to DEF, and BCA to ĚFD; 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, namely, BC to EF; the other sides shall be equal, each to each, namely, AB to DE, and AC to DF; and the third angle BAC to the third angle EDF.

D

B

C E