$72 a^2=b^3 \\ 8.9 a^2=b^3 \\ 2^3.3^2.a^2=b^3 \\ a=81=3^4 \Rightarrow b^3=2^3.3^2.3^4=2^3.3^6 \rightarrow b=2.3^2=18 \\ \{a,b\}_{min} \in N^+=\{81,18\}$