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and the angle ACF to the alternate angle CAD; therefore Book VI. also EAD is equal to the angle CAD, Wherefore, if the outward, &c. Q.E.D.

PROP. IV. THEOR.

THE sides about the equal angles of equiangular triangles are proportionals; and those which are opposite to the equal angles are homologous sides, that is, are the antecedents or consequents of the ratios.

a

a

Let ABC, DCE be equiangular triangles, having the angle ABC equal to the angle DCE, and the angle ACB to the angle DEC; and consequently the angle BAC equal to the * 32. 1. angle CDE. The sides about the equal angles of the triangles ABC, DCE are proportionals; and those are the homologous sides which are opposite to the equal angles. Let the triangle DCE be placed, so that its side CE may be contiguous to BC, and in the same straight line with it: And because the angles ABC, ACB are together less than two right angles b, ABC, and DEC, which is equal to ACB, are also less than two right angles; wherefore BA, ED produced shall meet; let them be produced and meet in the point F; and because the angle ABC is equal to the angle DCE, BF is BL parallel to CD. Again, because

A

17. 1.

D

• 12 Ax. 1.

d 28. 1.

the angle ACB is equal to the angle DEC, AC is parallel to FE: Therefore FACD is a parallelogram, and consequently AF is equal to CD, and AC to FD: And because 34. 1. AC is parallel to FE, one of the sides of the triangle FBE, BA is to AF, as BC to CEf: But AF is equal to CD; 2. 6, therefore, as BA to CD, so is BC to CE; and alternately, 8 7. 5. ́as AB to BC, so is DC to CE: Again, because CD is parallel to BF, as BC to CE, so is FD to DEf; but FD is equal to AC; therefore, as BC to CE, so is AC to DE: And alternately, as BC to CA, so CE to DE: Therefore, because it has been proved that AB is to BC, as DC to CE, and as BC to CA, so CE to ED, ex æqualih, BA is to AC, 2.5. as CD to DE. Therefore the sides, &c. Q. E. D.

Book VI

PROP. V. THEOR.

If the sides of two triangles, about each of their angles, be proportionals, the triangles shall be equiangular, and have their equal angles opposite to the homologous sides.

Let the triangles ABC, DEF have their sides propor tionals, so that AB is to BC, as DE to EF: and BC to CA, as EF to FD; and consequently, ex æquali, BA to AC, as ED to DF; the triangle ABC is equiangular to the triangle DEF, and their equal angles are opposite to the homologous sides, viz. the angle ABC equal to the angle DEF, and BCA to EFD, and also BAC to EDF.

a 23. 1. At the points E, F, in the straight line EF, make the
angle FEG equal to the angle ABC, and the angle EFG
equal to BCA; wherefore the
remaining angle BAC is equal

32. 1. to the remaining angle EGF,
and the triangle ABC is there-
fore equiangular to the triangle
GEF; and consequently they B
have their sides opposite to the
4. 6. equal angles proportionals.
Wherefore, as AB to BC, so is

A

E

GE to EF; but as AB to BC, so is DE to EF; therefore 11. 5. as DE to EF, sod GE to EF: Therefore DE and GE have 9. 5. the same ratio to EF, and consequently are equale: For

the same reason DF is equal to FG: And because, in the triangles DEF, GEF, DE is equal to EG, and EF common, the two sides DE, EF, are equal to the two GE, EF, and the base DF is equal to the base GF; therefore the angle 18. 1. DEF is equal to the angle GEF, and the other angles to 4. 1. the other angles which are subtended by the equal sides. Wherefore the angle DFE is equal to the angle GFE, and EDF to EGF: And because the angle DEF is equal to the angle GEF, and GEF to the angle ABC; therefore the angle ABC is equal to the angle DEF: For the same reason, the angle ACB is equal to the angle DFE, and the angle at A, to the angle at D. Therefore the triangle ABC is equiangular to the triangle DEF. Wherefore, if the sides, &c. Q.E.D.

Book VI.

PROP. VI. THEOR.

IF two triangles have one angle of the one equal to one angle of the other, and the sides about the equal angles proportionals, the triangles shall be equiangular, and shall have those angles equal which are opposite to the homologous sides.

Let the triangles ABC, DEF have the angle BAC in the one equal to the angle EDF in the other, and the sides about those angles proportionals; that is, BA to AC, as ED to DF: the triangles ABC, DEF are equiangular, and have the angle ABC equal to the angle DEF, and ACB to DFE.

A

D

32. 1.

At the points D, F, in the straight line DF, makea the a 23. 1. -angle FDG equal to either of the angles BAC, EDF; and the angle DFG equal to the angle ACB: Wherefore the remaining angle at B is equal to the remaining one at Gb, and consequently the triangle ABC is equiangular to the triangle DGF; and therefore as BA to AC, 3

7

E

e

so is GD to DF; but by the hypothesis, as BA to AC, so € 4. 6. is ED to DF; as therefore ED to DF, so isd GD to DF; 11. 5. ·wherefore ED is equale to DG; and DF is common to the 9. 5. two triangles EDF, GDF: Therefore the two sides ED, DF, are equal to the two sides GD, DF; and the angle EDF is equal to the angle GDF; wherefore the base EF is equal to the base FG, and the triangle EDF to the tri- f4. 1. angle GDF, and the remaining angles to the remaining angles, each to each, which are subtended by the equal sides; Therefore the angle DFG is equal to the angle DFE, and the angle at G to the angle at E: But the angle DFG is equal to the angle ACB; therefore the angle ACB is equal to the angle DFE: And the angle BAC is equal to the angle EDFs; wherefore also the remaining angle ate Hyp. B is equal to the remaining angle at E. Therefore the triangle ABC is equiangular to the triangle DEF. Wherefore, if two triangles, &c. Q.E.D.

BOOK VI.

PROP. VII. THEOR.

See N. IF two triangles have one angle of the one equal to one angle of the other, and the sides about two other angles proportionals, then, if each of the remaining angles be either less, or not less, than a right angle; or if one of them be a right angle: The triangles shall be equiangular, and have those angles equal about which the sides are proportionals.

Let the two triangles ABC, DEF have one angle in the one equal to one angle in the other, viz. the angle BAC to the angle EDF, and the sides about two other angles ABC, DEF proportionals, so that AB is to BC, as DE to EF; and, in the first case, let each of the remaining angles at C, F be less than a right angle. The triangle ABC is equiangular to the triangle DEF, viz. the angle ABC is equal to the angle DEF, and the remaining angle at C to the remaining angle at F.

A

For if the angles ABC, DEF be not equal, one of them is greater than the other: Let ABC be the greater, and at the point B, in the straight line AB, make the angle. 23. 1. ABG equal to the angle a DEF: And because the angle at A is equal to the angleB at D, and the angle ABG to

G

C E

F

32. 1. the angle DEF; the remaining angle AGB is equal to the remaining angle DFE: Therefore the triangle ABG is 4. 6. equiangular to the triangle DEF; wherefore as AB is to

BG, so is DE to EF; but as DE to EF, so, by hypothesis, 11. 5. is AB to BC; therefore as AB to BC, so is AB to BGd:

And because AB has the same ratio to each of the lines BC, 9. 5. BG; BC is equale to BG, and therefore the angle BGC is 15. 1. equal to the angle BCG: But the angle BCG is, by hy

pothesis, less than a right angle; therefore also the angle BGC is less than a right angle, and the adjacent angle 131. AGB must be greater than a right angles. But it was proved that the angle AGB is equal to the angle at F; therefore the angle at F is greater than a right angle: But,

by the hypothesis, it is less than a right angle; which is Book VI. absurd. Therefore the angles ABC, DEF are not unequal, that is, they are equal: And the angle at A is equal to the angle at D; wherefore the remaining angle at C is equal to the remaining angle at F: Therefore the triangle ABC is equiangular to the triangle DEF.

Next, Let each of the angles at C, F be not less than a right angle: The triangle ABC is also in this case equiangular to the triangle DEF...

B

C

G

F

The same construction being made, it may be proved in like manner that BC is equal to BG, and the angle at C equal to the angle BGC: But the angle at C is not less than a right angle; therefore the angle BGC is not less than a right angle: Wherefore two angles of the triangle BGC, are together not less than two right angles, which is impossible; and therefore the triangle ABC may be proved to 17. I, be equiangular to the triangle DEF, as in the first case.

Lastly, Let one of the angles at C, F, viz, the angle at C, be a right angle; in this case likewise the triangle ABC is equiangular to the triangle DEF.

For if they be not equiangular, make, at the point B of the straight line AB, the angle ABG equal to the angle DEF; then it may be proved, as in the first case, that BG is equal to BC: But the angle BCG is a right angle, therefore the angle BGC is also a right angle; whence two of the angles of the triangle BGC are together not less than two

B

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C

G

right angles, which is impossible: Therefore the triangle ABC is equiangular to the triangle DEF. Wherefore, if the two triangles, &c. Q. E. D.

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