Who first proved that every triangle has a circumscribed circle?

Question

Wikipedia only mentions that it follows from the Cartesian equation for a circle: $\left(x - a \right)^2 + \left( y - b \right)^2=r^2$

https://en.wikipedia.org/wiki/Circumscribed_circle#cite_note-1

but I'm interested to know how far back this dates.

Answer 1

It is Proposition $5$ in Book IV of Euclid's Elements.

Undoubtedly the result predates Euclid by many years, indeed centuries. But little pre-Euclidean material has survived.