= Solution
In a <face-centered cubic lattice>, nearest neighbours touch along a face diagonal. Four radii span that diagonal, so $\sqrt2\,a=4r$ and
$$
\boxed{a=2\sqrt2\,r.}
$$
Thus
$$
\boxed{a_{\mathrm{Cu}}=3.61473\ \text{Å},\qquad
a_{\mathrm{Ag}}=4.08425\ \text{Å}.}
$$
The primitive vectors enclose volume $a^3/4$, containing one atom; the conventional cubic cell of volume $a^3$ contains four. Keeping these cell conventions distinct avoids a factor-of-four error in the electron density.
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