Higbee, F. G · New York, John Wiley & Sons · leaf 97
Fig. 67. … Fig. 68. apex. In discussions of conical surfaces in this text only onenappe will be considered, however. When the plane curve SURFACES 87 directrix is a closed curve the surface is called a cone. In Fig. 68the generatrix moves so that it goes through the point O, the apex,and constantly touches…
Geometry, Descriptive
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