= Solution
An encounter can occur only near an <orbital node> of the comet's inclined orbit. Its tangential velocity projected into the planet's plane is reduced by $\cos I_c$, so the relative-speed calculation becomes
$$
\boxed{\Delta v_{pc}^2=v_p^2\left[3-\frac{a_p}{a_c}
-2\sqrt{\frac{a_c}{a_p}(1-e_c^2)}\cos I_c\right]}.
$$
The missing projected component is a vertical relative velocity, so the moon encounter is three-dimensional: $b$ becomes a vector in the impact plane, and a collision also requires the trajectory to pass close to the moon's orbital plane. Once the total asymptotic speed is used, the two-body <gravitational focusing> formulae in parts (v) and (vi) retain their form, but the geometrical collision probability is lower.
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