In a triangle, a cevian is a line segment whit one endpoint at a vertex of the triangle and the other endpoint on the opposite side. Sometimes cevians come in sets of three, like the three medians of a triangle, the three angle bisectors, or the three altitudes. In each of these cases, the three cevians are concurrent: The medians meet at the centroid, the angle bisectors meet at incenter, and the altitudes meet at orthocenter.
In his 1678 work De lineis rectis se invicem secantibus statica constructio, the Italian mathematician Giovanni Ceva (1647-1734) presented the following result (FIGURE 1). We are using the same notation to represent both line segments and their lengths.