$A B C$ is a triangle, right angled at $A .$ The resultant of the
forces acting along $\overline{A B}, \overline{B C}$ with magnitudes $\frac{1}{A B}$ and
$\frac{1}{A C}$ respectively is the force along $\overline{A D},$ where $D$ is the
foot of the perpendicular from $A$ onto $B C$. The magnitude of the resultant is