Who discovered the nine point circle?
Many mathematicians have been associated with this theorem, but there is no "one" who can be called the father of the nine point circle. The main person you can say who discovered the proof was Karl Wilhelm Feuerbach. He was born on May 30th in Germany. He was noted for being quite studious and he excelled in academics. Unlike many other mathematicians, Feuerbach's career in math was quite short-lived, 6 years. His main area of study was geometry, and what he was most known for was his work with the nine point circle. He also proved that the nine point circle touches the inscribed circle as well as the three excircles, which are the circle that are tangent to the sides of the triangle. This property of the nine point circle was published in his paper in 1822.
" The circle which passes through the feet of the altitudes of a triangle touches all four of the circles which are tangent to the three sides of the triangle; it is internally tangent to the inscribed circle and externally tangent to each of the the circles which touch the sides of the triangle externally."
Not to say that Feuerbach was the first and last to do work on the nine point circle, there were also others who shared his interests: Brianchon and Poncelet also published a proof of this theorem a year before Feuerbach did.