what is the reciprocal

keeping in mind that, any expression, can be written as a rational with a denominator of 1, since 3 is really 3/1 and 3/1 is well, just 3, likewise, "anything" is just "anything"/1 and so on.

the reciprocal of a rational, is just the same thing, upside-down, thus

[tex]\bf u^2-s^2\implies \cfrac{u^2-s^2}{1}\implies \stackrel{reciprocal}{\cfrac{1}{u^2-s^2}}[/tex]