2. On a line, four points $A, B, C$, $D$ are chosen, such that $A B=1, B C=2, C D=4$. What can $A D$ be? List all possibilities.
$\triangleright$ First, let's look at what the distance between points $A$ and $C$ might be. As in the previous problem, there are two possibilities (point $B$ is inside $A C$ or outside) - and it turns out to be either 3 or 1. Now we get two problems: in one, $A C=3$ and $C D=4$, and in the other, $-A C=1, C D=4$. Each has two solutions, so we get four answers in total: $4+3, 4-3, 4+1$, and $4-1$. Answer: the distance $A D$ can be $1, 3, 5$ or $7. \triangleleft$