Median to Hypotenuse of Right Triangle equals Half Hypotenuse
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Theorem
Let $\triangle ABC$ be a right triangle such that $BC$ is the hypotenuse.
Let $AD$ be the median to $BC$.
Then $AD$ is half of $BC$.
Proof
Construct $BE$ and $CE$ parallel to $AC$ and $AB$ respectively.
Then by definition $ABEC$ is a parallelogram.
By construction, $BC$ is a diagonal of $ABEC$ such that $AD$ is a bisector of it.
Thus by Quadrilateral is Parallelogram iff Diagonals Bisect each other, $AE$ is also a bisector of $ABEC$.
As $\angle BAC$ is a right angle it follows that $\angle BEC$ is also a right angle.
Thus by definition $ABEC$ is a rectangle.
From Diagonals of Rectangle are Equal:
- $AE = BC$
From above, $D$ is the midpoint of both $AE$ and $BC$.
Thus $AD = BD$ and hence the result.
$\blacksquare$
Also see
- Thales' Theorem, the converse of this.
Sources
- 1968: M.N. Aref and William Wernick: Problems & Solutions in Euclidean Geometry ... (previous) ... (next): Chapter $1$: Triangles and Polygons: Theorems and Corollaries $1.27$