What mathamatic symbol did Rene Descartes invent?

Answer 1

The one known by most the the self-named Descartes' Theorem, in where it explains the relationship of Four mutually tangent circles. Descartes' first addressed this theorem in a letter to Princess Elizabeth of Bohemia.

Answer 2

Descartes invented the symbol used to indicate a square root, known as the radical symbol, which is shaped like a check mark. He also came up with modern exponential notation.