The hinge theorem says that if two triangles 
and 
have congruent sides 
and 
and 
, then 
.
Hinge Theorem
This entry contributed by Floor van Lamoen
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van Lamoen, Floor. "Hinge Theorem." From MathWorld--A Wolfram Resource, created by Eric W. Weisstein. https://mathworld.wolfram.com/HingeTheorem.html