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vertex, the section is a triangle." In the case of a double cone, the section is two triangles such that the angles at the vertex are vertical angles.
* Proposition I.4 asserts that sections of a cone parallel to the base are circles with centers on the axis.
* Proposition I.13 defines the ellipse, which is conceived as the cutting of a single cone by a plane inclined to the plane of the base and intersecting the latter in a line perpendicular to the diameter extended of the base outside the cone (not

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