Sec. 1.5 Revision of geometrical concepts 11
A
B
Figure 1.3. A half-line[A,B.
A
B
C
Opposite half-lines.
- If the pointsB,Care distinct fromA, then the pair of half-lines
{[A,B,[A,C}is called anangle-supportand denoted by|BAC;if[A,B and[A,C
are opposite half-lines, then|BACis called astraightangle-support. In each caseAis
called itsvertex,[A,Band[A,Citsarms.
A
B
C
Figure 1.4. An angle-support.
A
B
C
A straight angle-support.
- The set of all the points on, or to one side of, a lineABis called aclosed
half-plane, withedge AB.
A
B
Figure 1.5. A closed half-plane shaded.
- If the pointsA,B,Care not collinear, then the set of points which are in both the
closed half-plane with edgeAB, containingC, and the closed half-plane with edge
AC, containingB, is called theinterior regionof|BACand denoted byIR(|BAC);
also(Π\IR(|BAC))∪|BACis called theexterior regionof|BACand denoted by
ER(|BAC).WhenC∈[A,Bthe interior and exterior regions of|BACare taken to be
[A,BandΠ, respectively.