= Solution
An <electron> accelerated from negligible initial <kinetic energy> through $V$ gains $eV$. The largest possible single <photon energy> in an X-ray tube is this entire <kinetic energy>, so the <Duane-Hunt law> gives
$$
\boxed{\lambda_{\min}=\frac{hc}{eV}=\frac{1239.841984\,\mathrm{eV\,nm}}{25000\,\mathrm{eV}}=0.0496\,\mathrm{nm}=4.96\times10^{-11}\,\mathrm m.}
$$
The massive anode can absorb the required recoil <momentum> with negligible recoil <energy>; most actual events produce lower-<energy> <bremsstrahlung> <photons>. The acceleration <energy> $eV$ is valid without approximating the electron speed nonrelativistically.
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