一、前言 
$$
a \geq 0, b \geq 0 \Rightarrow \sqrt{a} \cdot \sqrt{b}=?
$$
$$
a \geq 0, b>0 \Rightarrow \frac{\sqrt{a}}{\sqrt{b}}=?
$$
$$
a \geq 0, b \geq 0 \Rightarrow \sqrt{a} \cdot \sqrt{b}=?
$$
$$
a \geq 0, b>0 \Rightarrow \frac{\sqrt{a}}{\sqrt{b}}=?
$$