3-ча 1. A railway train passes by an observer in $t_{1}$ seconds, and at the same speed, it passes through a bridge of length $a$ meters in $t_{2}$ seconds. Find the length and speed of the train.
Solution 2. Let the length of the train be $l$ m. To pass over the bridge, the front carriage must enter the bridge, travel $a$ m on it, and then travel another $l$ m so that the last carriage leaves the bridge. As a result, we get the system of equations $l=t_{1} v, l+a=t_{2} v$. Solving it, we find $l=\frac{t_{1} a}{t_{2}-t_{1}}$ m and $v=\frac{a}{t_{2}-t_{1}}$ m/s.