Sides of Orthic Triangle of Acute Triangle

Theorem
Let $\triangle ABC$ be an acute triangle with sides $a$, $b$ and $c$ opposite vertices $A$, $B$ and $C$ respectively.

Let $\triangle DEF$ be the orthic triangle of $\triangle ABC$.

Then the sides of $\triangle DEF$ are $a \cos A$, $b \cos B$ and $c \cos C$.