Steinitz's lemma

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Let A, B, C, D, and E be the corners of a convex pentagon. If angles at B, C, and D are not decreased and at least one is increased and the polygon remains convex, then  distance AE is increased (Steinitz's lemma for convex pentagons).

P. R. Cromwell, Polyhedra, New York : Cambridge University Press, 1997 pp. 235 - 237.

Contributed by: Izidor Hafner