## Precalculus (6th Edition) Blitzer

$34.64\ cm$
Step 1. Based on the given information, we have $a=\frac{40}{2}=20, b=\frac{20}{2}=10$ Thus $c=\sqrt {a^2-b^2}=10\sqrt 3$ Step 2. The electrode is at the other focus; thus the distance between the electrode and the kidney stone is $2c=20\sqrt 3\approx34.64\ cm$