+ 
+ 
+ 
= 
+
+
+
=
surface: f(x,y,z)=0 or z=f(x,y)
1. cut in from x-axis, spin around x-axis. 2. cut in from y-axis, spin around y-axis.
3. cross section, cut off from x-axis, spin around x-axis. 4. cross section, cut off from y-axis, spin around y-axis.
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spin:
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