In geometry, a cevian is a line that intersects both a triangle's vertex, and also the side that is opposite to that vertex.^{[1]}^{[2]} Medians and angle bisectors are special cases of cevians. The name "cevian" comes from the Italian mathematician Giovanni Ceva, who proved a well-known theorem about cevians which also bears his name.^{[3]}
The length of a cevian can be determined by Stewart's theorem: in the diagram, the cevian length d is given by the formula
Less commonly, this is also represented by the mnemonic “a man and his dad put a bomb in the sink”, meaning
If the cevian happens to be a median (thus bisecting a side), its length can be determined from the formula
or
since
Hence in this case
If the cevian happens to be an angle bisector, its length obeys the formulas
and^{[5]}
and
where the semiperimeter s = (a+b+c)/2.
The side of length a is divided in the proportion b:c.
If the cevian happens to be an altitude and thus perpendicular to a side, its length obeys the formulas
and
where the semiperimeter s = (a+b+c) / 2.
There are various properties of the ratios of lengths formed by three cevians all passing through the same arbitrary interior point:^{[6]}^{: 177–188 } Referring to the diagram at right,
These last two properties are equivalent because summing the two equations gives the identity 1 + 1 + 1 = 3.
A splitter of a triangle is a cevian that bisects the perimeter. The three splitters concur at the Nagel point of the triangle.
Three of the area bisectors of a triangle are its medians, which connect the vertices to the opposite side midpoints. Thus a uniform-density triangle would in principle balance on a razor supporting any of the medians.
If from each vertex of a triangle two cevians are drawn so as to trisect the angle (divide it into three equal angles), then the six cevians intersect in pairs to form an equilateral triangle, called the Morley triangle.
Routh's theorem determines the ratio of the area of a given triangle to that of a triangle formed by the pairwise intersections of three cevians, one from each vertex.