# Median (geometry)

File:Triangle.Centroid.svg
The triangle medians and the centroid.

In geometry, a median of a triangle is a line segment joining a vertex to the midpoint of the opposing side. Every triangle has exactly three medians, one from each vertex, and they all intersect each other at the triangle's centroid. In the case of isosceles and equilateral triangles, a median bisects any angle at a vertex whose two adjacent sides are equal in length.

The concept of a median extends to tetrahedra.

## Relation to center of mass

Each median of a triangle passes through the triangle's centroid, which is the center of mass of an object of uniform density in the shape of the triangle.[1] Thus the object would balance on the intersection point of the medians.

## Equal-area division

Each median divides the area of the triangle in half; hence the name, and hence a triangular object of uniform density would balance on any median. (Any other lines which divide the area of the triangle into two equal parts do not pass through the centroid.)[2][3] The three medians divide the triangle into six smaller triangles of equal area.

### Proof of equal-area property

Consider a triangle ABC. Let D be the midpoint of ${\displaystyle {\overline {AB}}}$, E be the midpoint of ${\displaystyle {\overline {BC}}}$, F be the midpoint of ${\displaystyle {\overline {AC}}}$, and O be the centroid (most commonly denoted G).

By definition, ${\displaystyle AD=DB,AF=FC,BE=EC\,}$. Thus ${\displaystyle [ADO]=[BDO],[AFO]=[CFO],[BEO]=[CEO],}$ and ${\displaystyle [ABE]=[ACE]\,}$, where ${\displaystyle [ABC]}$ represents the area of triangle ${\displaystyle \triangle ABC}$ ; these hold because in each case the two triangles have bases of equal length and share a common altitude from the (extended) base, and a triangle's area equals one-half its base times its height.

We have:

${\displaystyle [ABO]=[ABE]-[BEO]\,}$
${\displaystyle [ACO]=[ACE]-[CEO]\,}$

## Formulas involving the medians' lengths

The lengths of the medians can be obtained from Apollonius' theorem as:

${\displaystyle m_{a}={\sqrt {\frac {2b^{2}+2c^{2}-a^{2}}{4}}},}$
${\displaystyle m_{b}={\sqrt {\frac {2a^{2}+2c^{2}-b^{2}}{4}}},}$
${\displaystyle m_{c}={\sqrt {\frac {2a^{2}+2b^{2}-c^{2}}{4}}},}$

where a, b and c are the sides of the triangle with respective medians ma, mb, and mc from their midpoints.

Thus we have the relationships:[4]

${\displaystyle a={\frac {2}{3}}{\sqrt {-m_{a}^{2}+2m_{b}^{2}+2m_{c}^{2}}}={\sqrt {2(b^{2}+c^{2})-4m_{a}^{2}}}={\sqrt {{\frac {b^{2}}{2}}-c^{2}+2m_{b}^{2}}}={\sqrt {{\frac {c^{2}}{2}}-b^{2}+2m_{c}^{2}}},}$
${\displaystyle b={\frac {2}{3}}{\sqrt {-m_{b}^{2}+2m_{a}^{2}+2m_{c}^{2}}}={\sqrt {2(a^{2}+c^{2})-4m_{b}^{2}}}={\sqrt {{\frac {a^{2}}{2}}-c^{2}+2m_{a}^{2}}}={\sqrt {{\frac {c^{2}}{2}}-a^{2}+2m_{c}^{2}}},}$
${\displaystyle c={\frac {2}{3}}{\sqrt {-m_{c}^{2}+2m_{b}^{2}+2m_{a}^{2}}}={\sqrt {2(b^{2}+a^{2})-4m_{c}^{2}}}={\sqrt {{\frac {b^{2}}{2}}-a^{2}+2m_{b}^{2}}}={\sqrt {{\frac {a^{2}}{2}}-b^{2}+2m_{a}^{2}}}.}$

## Other properties

The centroid divides each median into parts in the ratio 2:1, with the centroid being twice as close to the midpoint of a side as it is to the opposite vertex.

For any triangle,[5]

${\displaystyle {\tfrac {3}{4}}}$(perimeter) < sum of the medians < (perimeter of the triangle).

For any triangle with sides ${\displaystyle a,b,c}$ and medians ${\displaystyle m_{a},m_{b},m_{c}}$,[5]

${\displaystyle {\tfrac {3}{4}}(a^{2}+b^{2}+c^{2})=m_{a}^{2}+m_{b}^{2}+m_{c}^{2}.}$

The medians from sides of lengths a and b are perpendicular if and only if ${\displaystyle a^{2}+b^{2}=5c^{2}.}$[6]

The medians of a right triangle with hypotenuse c satisfy ${\displaystyle m_{a}^{2}+m_{b}^{2}=5m_{c}^{2}.}$

Any triangle's area T can be expressed in terms of its medians ${\displaystyle m_{a},m_{b}}$, and ${\displaystyle m_{c}}$ as follows. Denoting their semi-sum (ma + mb + mc)/2 as σ, we have[7]

${\displaystyle T={\frac {4}{3}}{\sqrt {\sigma (\sigma -m_{a})(\sigma -m_{b})(\sigma -m_{c})}}.}$

## Tetrahedron

A tetrahedron is a three-dimensional object having four triangular faces. A line segment joining a vertex of a tetrahedron with the centroid of the opposite face is called a median of the tetrahedron. There are four medians, and they are all concurrent at the centroid of the tetrahedron.[8]