To rationalise the denominator for #a/sqrtb# you multiply by #sqrtb/sqrtb# since this turns the #sqrtb# on the bottom into a #b#, and is the same as multiply by #1#.
#12/sqrt13*sqrt13/sqrt13=(12sqrt(13))/13#
Since #12/13# cannot be simplified, we leave it as #(12sqrt13)/13#