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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 shall be less than the other two sides of the triangle, but shall contain a greater angle.

Steps of the Demonstration.

1. Prove that BA + AC > BE + EC, that BE+EC > CD + DB,

that much more is BA + AC > CD + Db,

2.

3.

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PROPOSITION XXIII.

Problem. At a given point in a given straight line to make a rectilineal angle equal to a given

rectilineal angle.

A

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Proved by Proposition VIII.

PROPOSITION XXIV.

F

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 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 G be greater than the base of the other.

B

F

Steps of the Demonstration.

1. Prove that (in As ABC, DEG) base BC = base EG,

that

DFG DGF,

2.

3.

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4.

that much more EFG > EGF,

5.

that. EG or BC > EF.

PROPOSITION XXV.

(Argument ad absurdum).

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

A

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PROPOSITION XXVI.

(Argument ad absurdum).

Theorem. If two triangles have two angles of one equal to two angles of the other, each to each, and one side equal to one side; viz., either the sides adjacent to the equal angles, or opposite to the equal angles in each: then shall the

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Suppose that AB DE, and that a part of AB, as

BG DE,

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shows the supposition to be false; and :. that

AB DE,

3. that in As ABC, DEF base AC base DF, and LBAC EDF.

B H C E

Steps of the Demonstration to Case 2nd.

Suppose that BC EF, and that a part of BC, as

BH EF,

Then prove, on that supposition,

1. that in AS ABH, DEF

2. that

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BHABCA; i. e. the ex. ▲ BHA (of ▲ AHC) inter. and opp. 4 BCA; which shows the supposition to be false, and that BC EF,

3. prove that sc=DF, and BẠC = ▲ EDF.

PROPOSITION XXVII.

(Argument ad absurdum).

Theorem. If a straight line falling upon two other

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straight lines makes the al

ternate angles equal to one A another, these two straight lines shall be parallel.

F

E

B

Steps of the Demonstration.

Having made the supposition that ABCD, and that they meet as in G,

Prove, on that supposition,

1. that in the A EGF the exterior AEF = inter. and opp. EFD, which is impossible, and ..

the supposition is false.

2. that similarly AB and CD do not meet towards A and c,

3.

E

and that AB || CD.

PROPOSITION XXVIII.

Theorem. If a straight line falling upon two other straight lines, makes the exterior angle equal to the interior and opposite angle on the same side of the line, or makes the interior angles on the same side together equal to two right angles, the two D straight lines shall be parallel to one another.

F

To prove the 1st part of the proposition, show by Proposition XV. that the alternate ≤s are equal.

Steps of the Demonstration to part 2nd.

1. Prove that S AGH + BGHS BGH + GHD, that alternate AGH alternate GHD, and that AB || CD.

2.

3.

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