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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 sums are equal. 3. When equals are taken from equals the remainders are equal. 4. When equals are added to unequals the sums 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. The 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.
48. SYMBOLS AND ABBREVIATIONS.
Post. postulate. is (or are) equal to. Def. definition. Langle.
Ax. axiom. angles.
Hyp. hypothesis. A triangle.
Cor. corollary. A triangles.
Q. E. D. quod erat demonstranll parallel.
Q. E. F. quod erat faciendum. S parallelograms.
Adj. adjacent. I perpendicular.
Ext.-int. exterior-interior. Is perpendiculars.
Alt.-int. alternate-interior. rt. Z right angle.
Iden. identical. rt. right angles.
Cons. construction. > is (or are) greater than. Sup. supplementary.
< is (or are) less than. Sup. adj. supplementary-adjart. A right triangle.
cent. rt. A right triangles.
Ex. exercise. O circle.
+ increased by
49. When one straight line crosses another straight line the vertical angles are equal.
from each of these equals the common ZOC A. Then
In like manner we may prove
Q. E. D.
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.
Let the adjacent angles LOCA + 20CB= 2 rt. K.
We are to prove A C and C B in the same straight line.
$ 34 (being sup.-adj. € ).
LOCA+LOCB= 2 rt. A.
..LOCA + LOCF=2OCA + LOCB. Ax. 1.
Take away from each of these equals the common ZOCA.
..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.
PROPOSITION III. THEOREM. 52. A perpendicular measures the shortest distance from a point to a straight line.
Let A B be the given straight line, C the given point,
and C O the perpendicular.
We are to prove CO < any other line drawn from C to A B, as C F. Produce C O to E, making 0 E = C 0.
Draw E F. On A B as an axis, fold over O C F until it comes into the plane of 0 EF.
The line 0 C will take the direction of O E,
(since OC=0 E by cons.).
$ 18 (having their extremities in the same points).
..CF + FE= 2 C F,
2 C0 < 2 C F ..CO< CF.
Q. E. D.