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PROPOSITION XVII. THEOREM. 75. Two straight lines which are parallel to a third straight line are parallel to each other.
Let A B and C D be parallel to E F.
Draw HK I to E F.
..Z HOB= 2 HP D,
(each being a rt. Z).
§ 72 (when two straight lines are cut by a third straight line, if the ext. -int. $ be equal, the two lines are II).
Q. E. D.
PROPOSITION XVIII. THEOREM. : 76. Two parallel lines are everywhere equally distant from each other.
E M H ,
two points in A B, as E and H, let EF and HK
Draw MPI to A B. On MP as an axis, fold over the portion of the figure on the right of M P until it comes into the plane of the figure on the left.
M B will fall on MA,
the point H will fall on E,
H K will fall on EF,
Also, P D will fall on PC,
and the point K will fall on P C.
3.18 (their extremities being the same points).
Q. E. D.
PROPOSITION XIX. THEOREM. 77. Two angles whose sides are parallel, two and two, and lie in the same direction, or opposite directions, from their vertices, are equal.
/ H EL
and BC and EF respectively, parallel and lying
Produce (if necessary) two sides which are not ll until they intersect, as at H; then ZB=DHC,
§ 70 (being ext.-int. 6), and ZE= DHC,
§ 70 ..LB= 2 E.
Let Is B' and E' (Fig. 2) have B' A' and E' D', and B'C'
and E' F' respectively, parallel and lying in oppo-
Produce (if necessary) two sides which are not || until they
§ 70 (being ext.-int. ), and
ZE = E' H'C',
(being alt.-int. A ) ;
78. If two angles have two sides parallel and lying in the same direction from their vertices, while the other two sides are parallel and lie in opposite directions, then the two angles are supplements of each other.
Let A B C and D E F be two angles having B C and ED
parallel and lying in the same direction from their vertices, while E F and B A are parallel and lie in opposite directions.
We are to prove Z A B C and Z D E F supplements of each other.
Produce (if necessary) two sides which are not || until they intersect as at II. 2 A B C = B IH D,
$ 70 (being ext.-int. £). Z DEF= 2 B II E,
§ 68 (being alt.-int. 4). But Z BI D and Z B II E are supplements of each other, § 34
(being sup.-adj. €). .. LA B C and Z D E F, the equals of Z BII D and ZBH E, are supplements of each other.
Q. E. D.
79. DEF. A Triangle is a plane figure bounded by three straight lines.
A triangle has six parts, three sides and three angles.
80. When the six parts of one triangle are equal to the six parts of another triangle, each to each, the triangles are said to be equal in all respects.
81. DEF. In two equal triangles, the equal angles are called Homologous angles, and the equal sides are called Homologous sides.
82. In equal triangles the equal sides are opposite the equal angles.
83. DEF. A Scalene triangle is one of which no two sides are equal.
84. DEF. An Isosceles triangle is one of which two sides are equal.
85. DEF. An Equilateral triangle is one of which the three sides are equal.
86. DEF. The Base of a triangle is the side on which the triangle is supposed to stand.
In an isosceles triangle, the side which is not one of the equal sides is considered the base.