= Solution
In the <tetragonal crystal system>, physical squared distances use $a^2(\Delta x^2+\Delta y^2)+c^2\Delta z^2$. Two Ti–O distances from the corner Ti have length $\sqrt2ua$; the other four have length $\sqrt{2a^2(1/2-u)^2+c^2/4}$. Equating both to $r$ gives
$$
a=\frac{r}{\sqrt2u},\qquad c=2r\sqrt{1-\frac{(1/2-u)^2}{u^2}}.
$$
Thus, within the equal-bond model,
$$
\boxed{a=4.596\,\mathrm{\mathring A},\qquad c=2.907\,\mathrm{\mathring A}}.
$$
The <rutile structure> remains tetragonal and its octahedral angles remain distorted; these numbers are not a claim about measured ambient-pressure lattice parameters.
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