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PROP. A. THEOR.

If the outward angle of a triangle made by producing one of its sides, be divided into two equal angles, by a straight line which also cuts the base produced; the segments between the dividing line and the extremities of the base have the same ratio which the other sides of the triangle have to one another: and if the segments of the base produced have the same ratio which the other sides of the triangle have, the straight line drawn from the vertex to the point of section divides the outward angle of the triangle into two equal angles.

A

Let the outward angle CAE of any triangle ABC be divided into two equal angles by the straight line AD which meets the base produced in D: BD is to DC, as BA to AC.

Through C draw CF parallel to AD (31. 1.): and because the straight line AC meets the parallels AD, FC, the angle ACF is equal to the alternate angle CAD (29. 1.): but CAD is equal to the angle DAE (Hyp.); therefore also DAE is equal to the angle ACF. Again, because the straight line FAE meets the parallels AD, FC, the outward angle DAE is equal to the

E inward and opposite angle CFA : but the angle ACF has been proved equal to the angle DAE; therefore also the angle ACF is equal to the angle CFA, and consequently the side AF is equal to the side AC (6. 1.): and because

B
С

D
AD is parallel to FC, a side of the
triangle BCF, BD is to DC, as BA to AF (2. 6.): but AF is equal to
AC; as therefore BD is to DC, so is BA to AC.

Let now BD be to DC, as BA to AC, and join AD; the angle CAD is equal to the angle DAE.

The same construction being made, because BD is to DC, as BA to AC; and that BD is also to DC, as BA to AF (11. 5.): therefore BA is to AC, as BA to AF (9. 5.); wherefore AC is equal to AF (5. 1.), and the angle AFC equal (5. 1.) to the angle ACF; but the angle AFC is equal to the outward angle EAD, and the angle ACF to the alternate angle CAD: therefore 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.

D

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 (32. 1.) the angle BAC equal to the 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 (17. 1.), ABC, and DEC, which is equal F to ACB, are also less than two right angles; wherefore BA, ED produced shall meet (12. Ax. 1.); let them be A produced and meet in the point F; and because the angle ABC is equal to the angle DCE, BF is parallel (28. 1.) to CD. Again, because the angle * ACB is equal to the angle DEC, AC B

с is parallel to FE (28. 1.): therefore FACD is a parallelogram ; and consequently AF is equal to CD, and AC to FD (34. 1.): and because AC is parallel to FE, one of the sides of the triangle FBE, BA is to AF, as BC to CE (2. 6.): but AF is equal to CD; therefore (7. 5.), as BA to CD, so is BC to CE; and alternately, as AB to BC so is DC to CE, (17. 1.): again, because CD is parallel to BF, as BC to CE, so is FD to DE (2. 6.): 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 æquali, (22. 5.) BA is to AC, as CD to DE. Therefore the sides, &c. Q. E. D.

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 proportionals, 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.

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

A

D angle BAC is equal to the remaining angle EGF (32. 1.), and the triangle ABC is therefore

ES

F equiangular to the triangle GEF; and consequently they have their sides opposite to the B

C. equal angles proportionals (4. 6.). Wherefore, as AB to BC, so is GE to EF; but as AB to BC, so is DE to EF; therefore as DE to EF, so (11. 5.) GE to EF: therefore DE and GE have the same ratio to EF, and consequently are equal (9. 5.): 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 DEF is equal (8. 1.) to the angle GEF, and the other angles to the other angles which are subtended by the equal sides (4. 1.): 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.

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.

At the points D, F, in the straight line DF, make (23. 1.) the angle FDG equal to either of the angles BAC, EDF; and the angle DFG equal to the angle ACB; wherefore A the remaining angle at B is equal

D

G to the remaining one at G (32. 1.), and, consequently the triangle ABC is equiangular to the triangle DGF; and therefore as BA to AC, so is (4. 6.) GD to DF: but, by the hypothesis, as BA to AC, so is ED to DF; 1 as therefore ED to DF, so is (11.5.) B

CE

F GD to DF; wherefore ED is equal (9. 5.) to DG; and DF is common to the 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 (4. 1.), and the triangle EDF to the triangle 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 EDF (Hyp.); wherefore also the remaining angle at B is equal to the remaining angle at E. Therefore the triangle ABC is equiangular to the triangle DEF. Wherefore, if two trianglee, &c. Q. E. D.

PROP. VII. THEOR.

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.

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

A ABG equal to the angle (23. 1.)

D DEF: and because the angle at A is equal to the angle at D, and

G the angle ABG to the angle DEF; the remaining angle AGB is equal B

С E

F (32. 1.) to the remaining angle DFE: therefore the triangle ABG is equiangular to the triangle DEF; wherefore, (4. 6.) as AB is to BG, so is DE to EF; but as DE to EF, so, by hypothesis, is AB to BC ; therefore as AB to BC, so is AB to BG (11. 5.); and because AB has the same ratio to each of the lines BC, BG; BC is equal (9. 5.) to BG, and therefore the angle BGC is equal to the angle BCG (5. 1.); but the angle BCG is, by hypothesis, less than a right angle; therefore also the angle BGC is less than a right angle, and the adjacent angle AGB must be greater than a right angle (13. 1.). 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 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.

* See Note.

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. The same construction being

A made, it may be proved in like

D manner that BC is equal to BG, and the angle at C equal to the

G angle BGC: but the angle at C is not less than a right angle; therefore the angle BGC is not less B

CE than a right angle; wherefore two angles of the triangle BGC are together not less than two right angles, which is impossible (17. 1.): and therefore the triangle ABC may be proved to 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 equiangu

А Jar, 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; B

D but the angle BCG is a right angle, therefore (5. 1.) the angle

A BGC is also a right angle; whence two of the angles of the triangle BGC are together not

E

F less than two right angles, which is impossible (17. 1.); therefore

B

с the triangle ABC is equiangular to the triangle DEF. Wherefore, if two triangles, &c. Q. E. D.

С

G

PROP. VIII. THEOR.

In a right angled triangle, if a perpendicular be drawn from the right angle to the base, the triangles on each side of it are similar to the whole triangle, and to one another.*

*See Note.

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