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PROPOSITION XXIX.

(Argument ad absurdum).`

E

G

Theorem. If a straight line fall upon two parallel straight lines, it makes the alternate angles equal to one another, and the exterior angle equal to the interior and opposite upon the same side; and the two interior angles upon the same side together equal to two right angles.

A

Steps of the Demonstration.

Suppose that AGH

1. that

H

GHD, and that AGH > GHD,

Then prove, on that supposition,

AGH + BGH > <S BGH + GHD,

2. that S BGH + GHD < 2 rights,

3. that. AB, CD would meet if produced far enough, which shows the supposition to be

false,

and

that LAGH = GHD.

4. Prove that LS EGB=GHD,

5.

6.

that S EGB + BGHS BGH + GHD, that LS BGH + GHD 2 right Ls.

* The proof of this proposition depends on what is called the 12th Axiom, which, however, is so far from being an Axiom, that it is a Theorem quite as much in need of demonstration as that which it is here brought forward to establish. The author is happy to state that the following proof of it has been pronounced to be generally satisfactory by most competent authority :

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Proved by showing that the alternate S AGK, Gkd are equal.

PROPOSITION.

"If a right line meet two right lines, so as to make the two interior angles on the same side of it taken together less than two right angles, these right lines being continually produced, shall at length meet on that side on which are the angles which are less than two right angles."

Let the right line EF meet the two right lines AB, CD; and let the S BKL, KLD be

VE

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Since lines which, when they are produced ever so far, and do not meet, are parallel lines; it is plain that lines which are not parallel will meet, if produced sufficiently; for, if not, they would be parallel by the definition.

Now, let GH, a line passing through the point κ in which EF cuts AB be such that s HKL + KLD = 2 rights, .. by Proposition XXVIII. GH || CD,

*

.. AB is not || CD.

For it cuts GH, which is || CD, in the point K,

* An axiom is here implied, which is as self-evident as any employed in Euclid; viz., that through the same point there cannot be drawn two straight lines which shall both be parallel to the same straight line.

E

B D

PROPOSITION XXXI.

Problem. To draw a straight line through a given point parallel to a given straight line.

PROPOSITION XXXII.

Theorem. If a side of any triangle be produced,

the exterior angle is equal to the two interior and opposite angles; and the three interior angles of every triangle are equal to two right angles.

Steps of the Demonstration.

1. Prove that the altern.

BAC alternate ACE,

And since AB, CD are not parallels, they will meet.

E

Demon.

Again, AB and CD will meet on that side of EF on which are thes which are < 2 rights;

For suppose them to meet on the other side of EF, as in M, then KML would be a A;

And since S AKL + BKL = 2 right Дs,

and S CLK + DLK = 2 right Zs,

13. 1

.. the 4 S AKL + CLK + BKL + DLK = 4 rights,
of which S BKL + DLK < 2 right ≤s, Hypoth.
..S AKL + CLK > 2 rights, and these,

on the above supposition,

are 2 s of the ▲ KмL, which is impossible;

and the supposition is false;

.. AB, CD do not meet towards a and c ;
but it has been proved that they will meet,
.. they must meet towards the points в and D;
Wherefore, if a right line, &c. &c. Q. E. D.

17. 1

2. Prove that exterior

3.

4.

5.

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that whole ex. ▲ ACD = 2 int. & opp. 4 s,

CAB + ABC,

that S ACD + ACB 4S CAB + ABC + BCA,

that LS ABC + BCA + CAB = 2 rt. Ls.

Obs. The two corollaries which follow this proposition are of great use, and therefore necessary to be learned; there is one point in the first which may appear strange to a beginner, viz., that the s at the vertex F, are said to be equal to 4 rights; but by inspecting the annexed figure he will perceive that the s above the straight line MN are together equal to 2 right

E

C

M

N

Zs (by Prop. XIII. 1.), and also the s below MN are together equal to 2 rights (by the same Proposition.)

Hence all the ZS EFM + DFE + CFD + NFC + BFN + AFB + MFA = 4 rights,

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s round the

. when the line мN is removed, the point F will still be together= 4rt. s.

PROPOSITION XXXIII.

Theorem. The straight lines A which join the extremities of equal and parallel straight lines towards the same parts are also themselves equal and parallel.

B

D

Steps of the Demonstration.

1. Prove that

ABC BCD,

that (in AS ABC, BCD) base AC = base BD, and

2.

3.

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Steps of the Demonstration.

1. Prove that in AS ABC, BCD, ZS ABC, BCA = S

BCD, CBD, ea. to ea., the adjacent side

BC being common,

that :. AB, AC = CD, DB each to each, and

2.

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Theorem. Parallelograms upon the same base, and between the same parallels, are equal to each other.

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