Bottema's theorem

Bottema's theorem is a theorem in plane geometry by the Dutch mathematician Oene Bottema (Groningen, 1901–1992).^[1]

Bottema's theorem construction; changing the location of vertex ${\textstyle C}$ changes the locations of vertices ${\textstyle E}$ and ${\textstyle F}$ but does not change the location of their midpoint ${\textstyle M}$

The theorem can be stated as follows: in any given triangle ${\textstyle ABC}$, construct squares on any two adjacent sides, for example ${\textstyle AC}$ and ${\textstyle BC}$. The midpoint of the line segment that connects the vertices of the squares opposite the common vertex, ${\textstyle C}$, of the two sides of the triangle is independent of the location of ${\textstyle C}$.^[2]

The theorem is true when the squares are constructed in one of the following ways:

• Looking at the figure, starting from the lower left vertex, ${\textstyle A}$, follow the triangle vertices clockwise and construct the squares to the left of the sides of the triangle.
• Follow the triangle in the same way and construct the squares to the right of the sides of the triangle.

If ${\textstyle S}$ is the projection of ${\textstyle M}$ onto ${\textstyle AB}$, Then ${\textstyle AS=BS=MS}$.

If the squares are replaced by regular polygons of the same type, then a generalized Bottema theorem is obtained: ^[3]

In any given triangle ${\textstyle ABC}$ construct two regular polygons on two sides ${\textstyle AC}$ and ${\textstyle BC}$. Take the points ${\displaystyle D_{1}}$ and ${\displaystyle D_{2}}$ on the circumcircles of the polygons, which are diametrically opposed of the common vertex ${\textstyle C}$. Then, the midpoint of the line segment ${\displaystyle D_{1}D_{2}}$ is independent of the location of ${\textstyle C}$.

See also

• Van Aubel's theorem
• Napoleon's theorem