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BOOK VI.

DEFINITIONS.

I.

SIMILAR rectilineal figures are those which have their several angles equal, each to each, and the sides about the equal angles proportionals.

II.

"Reciprocal figures, viz. triangles and parallelograms, are such as have their sides about two of their angles proportionals in such a manner, that a side of the first figure is to a side of the other, as the remaining side of the other is to the remaining side of the first."

III.

A straight line is said to be cut in extreme and mean ratio, when the whole is to the greater segment, as the greater segment is to the less.

IV.

The altitude of any figure is the straight line drawn from its vertex perpendicular to the base.

PROPOSITION I. THEOREM.

Triangles and parallelograms of the same altitude are one to the other as their bases.

Let the triangles ABC, ACD, and the parallelograms EC, CF, have the same altitude,

viz. the perpendicular drawn from the point A to BD or BD produced.

As the base BC is to the base CD, so shall the triangle ABC be to the triangle ACD,

and the parallelogram EC to the parallelogram CF.

E A F

HGB C D h L

Produce BD both ways to the points H, L,

and take any number of straight lines BG, GH, each equal to the base BC; (1. 3.)

and DK, KL, any number of them, each equal to the base CD; and join AG, AH, AK, AL.

Then, because CB, BG, GH, are all equal,

the triangles AHG, AGB, ABC, are all equal: (1. 38.) therefore, whatever multiple the base HC is of the base BC, the same multiple is the triangle AHC of the triangle ABC: for the same reason whatever multiple the base LC is of the base CD, the same multiple is the triangle ALC of the triangle ADC: and if the base HC be equal to the base CL,

the triangle AHC is also equal to the triangle ALC: (1. 38.) and if the base HC be greater than the base CL, likewise the triangle AHC is greater than the triangle ALC; and if less, less;

therefore since there are four magnitudes,

viz. the two bases BC, CD, and the two triangles ABC, ACD; and of the base BC, and the triangle ABC, the first and third, any equimultiples whatever have been taken,

viz. the base HC and the triangle AHC;

and of the base CD and the triangle ACD, the second and fourth, have been taken any equimultiples whatever,

viz. the base CL and the triangle ALC;

and since it has been shewn, that, if the base HC be greater than the base CL,

the triangle AHC is greater than the triangle ALC;

and if equal, equal; and if less, less;

therefore, as the base BC is to the base CD, so is the triangle ABC to the triangle ACD. (v. def. 5.)

And because the parallelogram CE is double of the triangle ABC, (I. 41.)

and the parallelogram CF double of the triangle ACD,

and that magnitudes have the same ratio which their equimultiples have; (v. 15.)

as the triangle ABC is to the triangle ACD, so is the parallelogram EC to the parallelogram CF;

and because it has been shewn, that, as the base BC is to the base CD, so is the triangle ABC to the triangle ACD;

and as the triangle ABC is to the triangle ACD, so is the parallelogram EC to the parallelogram CF;

therefore, as the base BC is to the base CD, so is the parallelogram EC to the parallelogram CF. (v. 11.)

Wherefore, triangles, &c.

Q.E.D.

COR. From this it is plain, that triangles and parallelograms that have equal altitudes, are to one another as their bases.

Let the figures be placed so as to have their bases in the same straight line; and having drawn perpendiculars from the vertices of the triangles to the bases, the straight line which joins the vertices is parallel to that in which their bases are, (1. 33.) because the perpendiculars are both equal and parallel to one another. (1. 28.) Then, if the same construction be made as in the proposition, the demonstration will be the same.

PROPOSITION II. THEOREM.

If a straight line be drawn parallel to one of the sides of a triangle, it shall cut the other sides, or these produced, proportionally and conversely, if the sides, or the sides produced, be cut proportionally, the straight line which joins the points of section shall be parallel to the remaining side of the triangle.

Let DE be drawn parallel to BC, one of the sides of the triangle ABC. Then BD shall be to DA, as CE to EA.

[blocks in formation]

Join BE, CD.

Then the triangle BDE is equal to the triangle CDE, (1. 37.) because they are on the same base DE, and between the same parallels DE, BC;

but ADE is another triangle;

and equal magnitudes have the same ratio to the same magnitude; (v. 7.)

therefore, as the triangle BDE is to the triangle ADE, so is the triangle CDE to the triangle ADE:

but as the triangle BDE to the triangle ADE, so is BD to DA, (vI. 1.) because, having the same altitude, viz. the perpendicular drawn from the point E to AB, they are to one another as their bases; and for the same reason, as the triangle CDE to the triangle ADE, so is CE to EA:

therefore, as BD to DA, so is CE to EA. (v. 11.)

Next, let the sides AB, AC of the triangle ABC, or these sides produced, be cut proportionally in the points D, E, that is, so that BD may be to DA as CE to EA, and join DE.

Then DE shall be parallel to BC.

The same construction being made,

because as BD to DA, so is CE to EA;

and as BD to DA, so is the triangle BDE to the triangle ADE; (VI. 1.) and as CE to EA, so is the triangle CDE to the triangle ADE; therefore the triangle BDE is to the triangle ADE, as the triangle CDE to the triangle ADE; (v. 11.)

that is, the triangles BDE, CDE have the same ratio to the triangle ADE:

therefore the triangle BDE is equal to the triangle CDE: (v. 9.) and they are on the same base DĚ:

but equal triangles on the same base and on the same side of it, are between the same parallels; (1. 39.)

therefore DE is parallel to BC.

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

[blocks in formation]

If the angle of a triangle be divided into two equal angles, by a straight line which also cuts the base; the segments of the base shall have the same ratio which the other sides of the triangle have to one another: and conversely, if the segments of the base have the same ratio which the other sides of the triangle have to one another; the straight line drawn from the vertex to the point of section, divides the vertical angle into two equal angles.

Let ABC be a triangle, and let the angle BAC be divided into two equal angles by the straight line AD.

Then BD shall be to DC, as BA to AC.

E

Through the point C draw CE parallel to DA, (I. 31.)
and let BA produced meet CE in E.

Because the straight line AC meets the parallels AD, EC, the angle ACE is equal to the alternate angle CAD: (1. 29.). but CAD, by the hypothesis, is equal to the angle BAD; wherefore BAD is equal to the angle ACE. (ax. 1.) Again, because the straight line BAE meets the parallels AD, EC, the outward angle BAD is equal to the inward and opposite angle AEC: (1. 29.)

but the angle ACE has been proved equal to the angle BAD; therefore also ACE is equal to the angle AEC, (ax. 1.)

and consequently, the side AE is equal to the side AC: (1. 6.) and because AD is drawn parallel to EC, one of the sides of the triangle BCE,

therefore BD is to DC, as BA to AE: (vI. 2.)

but AE is equal to AC;

therefore, as BD to DC, so is BA to AC. (v. 7.)

Next, let BD be to DC, as BA to AC, and join AD. Then the angle BAC shall be divided into two equal angles by the straight line AD.

The same construction being made;

because, as BD to DC, so is BA to AC;

and as BD to DC, so is BA to AE, because AD is parallel to EC; (VI. 2.)

therefore BA is to AC, as BA to AE: (v. 11.)

consequently AC is equal to AE, (v. 9.)

and therefore the angle AEC is equal to the angle ACE: (1. 5.) but the angle AEC is equal to the outward and opposite angle BAD; and the angle ACE is equal to the alternate angle CAD: (1. 29.) wherefore also the angle BAD is equal to the angle CAD; (ax. 1.) that is, the angle BAC is cut into two equal angles by the straight line AD.

Therefore, if the angle, &c. Q. E.D.

PROPOSITION A. THEOREM.

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 conversely, 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.

Let ABC be a triangle, and let one of its sides BA be produced to E; and let the outward angle CAE be divided into two equal angles by the straight line AD which meets the base produced in D. Then BD shall be to DC, as BA to AC.

[blocks in formation]

Through C draw CF parallel to AD: (1. 31.)

and because the straight line AC meets the parallels AD, FC, the angle ACF is equal to the alternate angle CAD: (1. 29.) but CAD is equal to the angle DAE; (hyp.)

therefore also DAE is equal to the angle ACF. (ax. 1.) Again, because the straight line FAE meets the parallels AD, FC, the outward angle DAE is equal to the inward and opposite angle CFA: (1. 29.)

but the angle ACF has been proved equal to the angle DAE; therefore also the angle ACF is equal to the angle CFA; (ax. 1.) and consequently the side AF is equal to the side AC: (1. 6.) and because AD is parallel to FC, a side of the triangle BCF, therefore BD is to DC, as BA to AF: (vi. 2.) but AF is equal to AC;

therefore, as BD is to DC, so is BA to AC. (v. 7.)
Next, let BD be to DC, as BA to AC, and join AD.
The angle CAD, shall be equal to the angle DAE.
The same construction being made,

because BD is to DC, as BA to AC;

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