Envelopes created by families of parabolas in the plane
摘要
In this paper, envelopes of a family of parabolas in the plane are investigated. For a given family of parabolas, we obtain a necessary and sufficient condition concerning if the family of parabolas creates an envelope. Furthermore, we study the relation among an envelope of a family of parabolas in the plane and an envelope of the family of directrixes corresponding to the family of parabolas.