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fore 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 equal † to the angle ABC, therefore the angle ABC + Constr. is equal † to the angle DEF: for the same reason, the angle † 1 Ax. ACB is equal to the angle DFE, and the angle at A equal to the angle at D: therefore the triangle ABC is equiangular to the triangle DEF. Wherefore, if the sides, &c. Q. E, D,

PROPOSITION 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 lo 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 shall be equiangular, and shall have the angle ABC equal to the angle DEF, and ACB to DFE.

At the points D, F, in the straight line DF, make * the .23. 1. angle FDG equal to either of the angles BAC, EDF; and the angle DFG equal to the angle ACB:

A wherefore the remaining angle at B is

D

G

* 32. 1. & equal to the remaining angle at G:

3 Ax. and consequently the triangle ABC is equiangular to the triangle DGF: there- B В CE fore as BA to AC, so is GD to DF: but by the hypothesis, as BA to AC, so is ED to DF; therefore as ED to DF, so is * GD to DF; wherefore ED is equal 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, each to each ; and the angle EDF is equal † to the Constr. angle GDF; wherefore the base EF is equal to the base FG*, 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 + Constr. angle ACB is equal + to the angle DFE: and the angle BAC + 1 Ax.

*

# 4. 6.

• 11. 5. * 9.5.

# 4. 1.

Нур. . + 32. 1. & 3 Ax

is equal to the angle EDF: 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 triangles, &c.

Q. E. D.

PROPOSITION VII.

See N.

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 shall have those angles equal about which the sides are proportionals.

• 23. 1.

+ Нур. .

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 shall be equiangular to the triangle DEF, viz. the angle ABC shall be equal to the angle DEF, and the remaining angle at C equal to the remaining angle at F.

For if the angles ABC, DEF be not equal, one of them must be greater than the other: let ABC be the greater, and at the point B, in the straight line AB *, make the angle ABG equal to the angle DEF; and because the angle at A is equal † to the

D angle at D, and the angle ABG to the angle DEF, the remaining angle AGB

E F is equal * to the remaining angle DFE: therefore the triangle ABG is equiangular to the triangle DEF: wherefore * as AB is to BG, so is DE to EF: but as DE to EF so by hypothesis, is AB to BC: therefore to BC, so is AB to BG: and because AB has the same ratio to each of the lines BC, BG ; BC is equal * to BG; and therefore the angle BGC is equal * to the angle BCG: but the angle BCG is, by hypothesis, less than a right angle; there. fore also the angle BGC is less than a right angle; and therefore the adjacent angle AGB must be greater

* than a right angle: but it was proved that the angle AGB is equal to the

A

B

• 32. 1. & 3 Ax.

• 4. 6.

• 11.5.

as AB

• 9. 5.

• 5. I.

• 13. 1.

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 + Hyp. C is equal + to the remaining angle at F: therefore the tri. + 32. 1. &

3 Ar. angle 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 shall also in this case be equi. angular to the triangle DEF.

The same construction being made, it may be proved in like manner, that BC is

D equal to BG, and therefore the angle at C equal to the angle BGC: but the angle at c is not less † than a right angle ; there

+ Hyp. fore 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 • 17. 1. 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 shall be equiangular to the triangle DEF.

For if they be not equiangular, at the point B, in the straight line AB, make the angle ABG equal to the angle DEF: then it may be proved, as in the first case, that BG is equal to BC; and therefore * the angle BCG equal to the

• 5. 1. angle BGC: but the angle BCG is a

B right + angle, therefore the angle BGC

+ Нур. is also a right angle; whence two of the

+1 Ax. angles of the triangle BGC are together, Bnot less than two right angles ; which is impossible *: therefore the triangle

. 17. 1. ABC is equiangular to the triangle DEF. Wherefore, if two triangles, &c.

ܐ

Q. E. D.

PROPOSITION VIII. THEOR.-In a right-angled triangle, if a perpendicular be Sce N.

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.

† 11 Ax

A

* 32. 1. & 3 Ax.

B

• 4. 6.
* I Def. 6.

Let ABC be a right-angled triangle, having the right angle BAC; and from the point A, let AD be drawn perpendicular to the base BC: the triangles ABD, ADC shall be similar to the whole triangle ABC, and to one another.

Because the angle BAC is equal to the angle ADB +, each of them being a right angle, and that the angle at B is common to the two triangles ABC, ABD, the remaining angle ACB is equal to the remaining angle BAD*: therefore the triangle ABC is equiangular to the triangle

D ABD, and the sides about their equal angles are proportionals *; wherefore the triangles are * similar: in the like manner it may be demonstrated, that the triangle ADC is equiangular and similar to the triangle ABCI. And the triangles ABD, ACD, being both equiangular and similar to ABC, are equiangular and similar to each other. Therefore, in a right-angled, &c. Q. E. D.

COR. From this it is manifest, that the perpendicular drawn from the right angle of a right-angled triangle to the base, is a mean proportional between the segments of the base, and also that each of the sides is a mean proportional between the base, and the segment of it adjacent to that side: because in the triangles BDA, ADC, BD is to DA *, as DA to DC; and in the triangles ABC, DBA, BC is to BA *, as BA to BD; and in the triangles ABC, ACD, BC is to CA *, as CA to CD,

• 4. 6. . 4. 6. # 4. 6.

PROPOSITION IX.

See N.

PROBLEM.– From a given straight line to cut off any part

required.

Let AB be the given straight line; it is required to cut off any part from it.

From the point A, draw a straight line AC, making any angle with AB; and in AC take any point D, and take AC the same multiple of AD, that AB is of the part which is to be cut off from it; join BC, and draw DE parallel to it : then AE shall be the part required to be cut off.

+) Ax † 4. 6. 7 1 Def.6.

The triangles ADB, ADC, being both equiangular to ABC, are equiangular f to each other ; therefore they have the sides about their equal + angles proportionals; therefore they are + similar to each other.

Because ED is parallel to one of the sides of the triangle
ABC, viz. to BC, as CD is to DA so is * BE to
EA ; and by composition *, CA is to AD, as BA А
to AE: but CA is a multiple + of AD; therefore *

ELD
BA is the same multiple of AE : whatever part,
therefore, AD is of AC, AE is the same part of
AB. Wherefore, from the straight line AB, the

B part required is cut off. Which was to be done.

* 2. 6. * 18. 5. + Constr.

D. 5.

PROPOSITION X.

PROB.— To divide a given straight line similarly to a given

divided straight line, that is, into parts that shall have the same ratios to one another, which the parts of the divided given straight line have.

F

Let AB be the straight line given to be divided, and AC the divided line; it is required to divide AB similarly to AC.

Let AC be divided in the points D, E; and let AB, AC be placed so as to contain any angle, and join BC; and through the points D, E, draw * DF, EG parallels to

31.1. it: AB shall be divided in the points F, G,

A similarly to AC.

Through D, draw DHK parallel to AB: D therefore each of the figures FH, HB is a pa

GO HAE rallelogram: wherefore DH is equal* to FG,

• 34. I.

B K and HK to GB: and because He is parallel to KC, one of the sides of the triangle DKC, as CE to ED, so is * KH to HD: but KH is equal to BG, and HD to GF; there- • 2. 6. fore, as CE to ED +, so is BC to GF: again, because FD is +7.5. parallel to GE, one of the sides of the triangle AGE, as ED to DA, so is + GF to FA: but it has been proved that CE is to + 2 6. ED, as BG to GF: therefore as CE is to ED, so is BG to GF, and as ED to DA, so GF to FA. Therefore, the given straight line AB is divided similarly to AC. Which was to be done.

PROPOSITION XI.

PROB.To find a third proportional to two given straight

lines.

Let AB, AC be the two given straight lines; it is required to find a third proportional to AB, AC.

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