Example 4. Given $\log _{18} 9=a(a \neq 2)$, $18^{b}=5$, find $\log _{38} 45$.
Analysis: $\because \log _{18} 9 \neq \log _{18} 18^{2}=2$, $\therefore \log _{18} 9 \neq 2$, so the condition $a \neq 2$ in the problem is redundant. The reason the problem setter added the condition $\mathrm{a} \neq 2$ is mainly to avoid the denominator of the result $\log _{38} 45=\frac{a+b}{2-a}$ being 0, but this concern is unnecessary.