= Solution
The source labels $L$ as the full chord from $P_1$ to $P_3$, with $L/2$ on each side of its midpoint. The <Right triangle> between the <Circle> centre, chord midpoint and endpoint gives
$$
(R-s)^2+(L/2)^2=R^2,\qquad
\boxed{R=\frac{L^2}{8s}+\frac s2\simeq\frac{L^2}{8s}.}
$$
For fixed geometry and field, the inverse <transverse momentum> is
$$
\frac1{p_T}\simeq\frac{8s}{0.299792458\,|q/e|BL^2}.
$$
If the <Measurement uncertainty>[uncertainty] is dominated by the central point, $\sigma_s\simeq\sigma_x$. Linear <propagation of uncertainty> gives
$$
\boxed{\sigma(1/p_T)\simeq\frac{8\sigma_x}{0.299792458\,|q/e|BL^2}.}
$$
Using $|q/e|=1$, $B=2\,\mathrm T$, $L=0.6\,\mathrm m$ and $\sigma_x=28.87\,\mu\mathrm m$,
$$
\boxed{\sigma(1/p_T)=1.070\times10^{-3}\,(\mathrm{GeV}/c)^{-1}.}
$$
The inverse-momentum resolution is approximately <momentum> independent in this spatial-error-only model; the relative <momentum> resolution grows as $p_T\sigma(1/p_T)$. Endpoint errors and <multiple scattering> are excluded by the stated assumptions.
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