Positive common ratio of a geometric sequence from two terms and its first term in standard form

All the terms of a geometric sequence \(u_1,u_2,u_3,\dots\) are positive, and its common ratio is \(r\). The third term is \(7.2\times10^4\) and the fifth term is \(1.8\times10^4\).

1. Find the value of \(r\).[3]
2. Find \(u_1\), expressing it as \(a\times10^k\) with \(1\leq a<10\) and \(k\in\mathbb{Z}\).[3]