Curves {envelope, curve}| can be tangential to all curve-family curves. Curve families can have equation f(x,y,a) = 0, for parameter a. Curve families have boundary curves. Eliminating parameter from function and taking partial differential with respect to parameter can find envelope.
Mathematical Sciences>Geometry>Plane>Curve>Kinds>Envelope
3-Geometry-Plane-Curve-Kinds-Envelope
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Date Modified: 2022.0224