A dissection fallacy discovered by Dudeney (1958). The same set of tangram pieces can apparently produce two different figures, one of which is a proper subset of the other. This seems true only at a first glance: in reality the area is the same in both cases, since in the left picture the missing foot is compensated by a larger body.
Tangram Paradox
See also
Dissection Fallacy, TangramThis entry contributed by Margherita Barile
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References
Dudeney, H. Amusements in Mathematics. New York: Dover, 1958.Referenced on Wolfram|Alpha
Tangram ParadoxCite this as:
Barile, Margherita. "Tangram Paradox." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein. https://mathworld.wolfram.com/TangramParadox.html