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40. DEF. An Axiom is a truth which is admitted without demonstration.

41. DEF. A Postulate is a problem which is admitted to be possible.

42. DEF. A Proposition is either a theorem or a problem. 43. DEF. A Corollary is a truth easily deduced from the proposition to which it is attached.

44. DEF. A Scholium is a remark upon some particular feature of a proposition.

45. DEF. An Hypothesis is a supposition made in the enunciation of a proposition, or in the course of a demonstration.

46. AXIOMS.

1. Things which are equal to the same thing are equal to each other.

2. When equals are added to equals the wholes are equal.

3. When equals are taken from equals the remainders are equal. 4. When equals are added to unequals the wholes are unequal. 5. When equals are taken from unequals the remainders are unequal.

6. Things which are double the same thing, or equal things, are equal to each other.

7. Things which are halves of the same thing, or of equal things, are equal to each other.

8. The whole is greater than any of its parts.

9. Every whole is equal to all its parts taken together.

47. POSTULATES.

Let it be granted

1. That a straight line can be drawn from any one point to any other point.

2. That a straight line can be produced to any distance, or can be terminated at any point.

3. That the circumference of a circle can be described about any centre, at any distance from that centre.

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ON PERPENDICULAR AND OBLIQUE LINES.

PROPOSITION I. THEOREM.

49. When one straight line crosses another straight line the vertical angles are equal.

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:. LOCA + LOCB=ZOCA + Z ACP.

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Ax. 1.

Take away from each of these equals the common OCA.

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50. COROLLARY. If two straight lines cut one another, the four angles which they make at the point of intersection are together equal to four right angles.

PROPOSITION II. THEOREM.

51. When the sum of two adjacent angles is equal to two right angles, their exterior sides form one and the same straight line.

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Let the adjacent angles LOCA + ZOC B = 2 rt. .

We are to prove A C and C B in the same straight line.

Suppose C F to be in the same straight line with A C.

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:. LOCA + ZOC F = LOCA+Z OC B.

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Hyp.

Ax. 1.

Take away from each of these equals the common ≤ O CA.

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.. C B and C F coincide, and cannot form two lines as represented in the figure.

.. A C and C B are in the same straight line.

Q. E. D.

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Let A B be the given straight line, C the given point, and CO the perpendicular.

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On A B as an axis, fold over O C F until it comes into the

plane of O E F.

The line OC will take the direction of O E,

(since ▲ COF= LEO F, each being a rt. ▲).

The point C will fall upon the point E,

(since 0 C = 0 E by cons.).

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But

CO+ OE < C F + FE,

(a straight line is the shortest distance between two points).

Substitute

and

2 CO for CO + O E,

2 CF for CF+ FE; then we have

2 CO < 2 CF.

..CO< C F.

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Cons.

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Q. E. D.

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