6 INTRODUCTION It is divided by its midpoint into two smaller arcs, of which one consists of those points M on the arc for which the arc AM is greater than MB, the other of points for which AM is smaller than MB. O Figure 6 9. Two points of a circle are diametrically opposite (A, B, Fig. 6) if the line which joins them passes through the center. This segment is called a diameter. It is clear that a diameter has a length twice that of a radius. A circle is clearly determined when we are given one of its diameters. Its center is then the midpoint of the diameter. A diameter AB divides a circle into two arcs, which are, respectively, the por- tions of the circle situated in the two half-planes determined by line AB. These two arcs are equal: we can make them coincide by superimposing the two half-planes in question (6). Thus we have two semicircles. In the same fashion, a disk is divided by a diameter into two equal parts, which can be superimposed in the same manner as the semicircles.
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