don steward
mathematics teaching 10 ~ 16

Saturday, 7 April 2012

Thebault's theorem

Victor Thebault is credited with this theorem (1930)

create two equilateral triangles
on two adjacent sides of a square

join the three points as shown

prove that this (red dotty) triangle must be an equilateral triangle

you can do this by considering:
angles and establishing congruence;

or by other methods

what happens if the two triangles are drawn inside rather than outside the square?

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