XVIII
5.1 Hierarchy of sets of numbers. Each set contains the solution
to the equation to its right. The symbol ’⊂’ means inclusion of
a set in a yet bigger set. Thus, each set is a subset of the set in
the next higher level. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
5.2 The re al axis. The arrow leading from zero to the negative
number x produces an angle of 180 degrees (or π) with the
positive part of the real axis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
5.3 The imaginary number i. The arrow leading from the origin to
i produces a right angle w ith the positive part of the real axis.
This angle is doubled in i
2
to produce the required result −1. . 49
5.4 The complex plane. The complex number a + bi is represented
by the point (a, b). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
5.5 Adding the complex numbers a + bi and c + di by the paral-
lelogram rule produces the result a + c + (b + d)i. . . . . . . . . . . . 52
5.6 Multiplying the two complex numbers cos(θ) + sin(θ)i and
cos(φ) + sin(φ)i results in the complex number cos(θ + φ) +
sin(θ + φ)i. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
6.1 Mapping the line segment AB onto its image A
0
B
00
. This map-
ping implies that A
0
B
00
< A
0
B
0
. . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
6.2 The angle vertexed at B is mapped onto the angle vertexed at
B
0
using one compass to store the distance BC
00
= BA
00
and
another compass to store the distance C
00
A
00
. . . . . . . . . . . . . . . . 67
6.3 The angle DEF is smaller than the angle ABC, because it can
be mapped onto the angle A
0
BC, which lies inside the angle
ABC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
6.4 The sum of the angle ABC and the angle DEF is obtained by
mapping the latter onto the new angle A
0
BA to produce the
joint angle A
0
BC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
6.5 Pairs of corresponding angles between the parallel lines a and
b: α = α
0
, β = β
0
, γ = γ
0
, and δ = δ
0
. . . . . . . . . . . . . . . . . . . . . . . . 69
6.6 Proving the reve rsed corresponding-angle theorem by contra-
diction: if a were not parallel to b, then one could draw another
line a
0
that would be parallel to b. . . . . . . . . . . . . . . . . . . . . . . . . . 70
6.7 Proving the uniqueness of the parallel line a by contradiction: if
it were not, then one could draw another line a
0
that is parallel
to b as well.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
6.8 Proving the corresponding-angle axiom by contradiction: if γ
were smaller than γ
0
, then one could draw another line a
0
so
that the new angle BOQ is equal to γ
0
. . . . . . . . . . . . . . . . . . . . . 72
6.9 The triangle 4ABC with the interior angles vertexed at A,
B, and C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
6.10 Proving that the sum of the angles in a triangle is always π,
using the alternate-angle theorem. . . . . . . . . . . . . . . . . . . . . . . . . . 74
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