√5 / 25

Write numerator and denominator in terms of a common base (let's use 5)

= 5^(1/2) / 5^2

In division, subtract the exponent in the denominator from the exponent in the numerator

= 5^(1/2 - 2)
= 5^(-3/2)

As John pointed out, this simplification is not an example of rationalizing the denominator, though I see where you might be confused.