Did Euclid discover how many degrees a triangle has?

Euclid did not discover that a triangle has 180 degrees; rather, he provided a logical framework for understanding this property in his work "Elements." The relationship of the interior angles of a triangle summing to 180 degrees was understood and proved within the context of Euclidean geometry. This concept was established long before Euclid, but his systematic treatment helped solidify its acceptance in mathematics.