Example 2. Two bus stations, $A$ and $B$, continuously send out a bus at the same intervals, with each bus traveling at the same speed. A cyclist on the road between $A$ and $B$ notices that a bus passes from behind every $a$ minutes and a bus passes from the front every $b$ minutes. How often do stations $A$ and $B$ send out a bus?
Let $A, B$ two stations send out a bus every $x$ minutes. According to the problem, we have $\left(\frac{1}{a}+\frac{1}{b}\right) x=2$.
Solving for $x$ gives $x=\frac{2 a b}{a+b}$.
Answer: $A, B$ two stations send out a bus every $\frac{2 a b}{a+b}$ minutes.