Events One of Which equals Intersection/Examples/Target of Concentric Circles

Examples of Use of Events One of Which equals Union
Let $T$ be a target which consists of $10$ concentric circles $C_1$ to $C_{10}$ whose radii are respectively $r_k$ for $k = 1, 2, \ldots, 10$.

Let $r_k < r_{k + 1}$ for all $k = 1, 2, \ldots, 9$.

That is, let $C_1$ be the innermost and $C_{10}$ be the outermost.

Let $A_k$ denote the event of hitting $T$ inside the circle of radius $r_k$.

Let $C$ denote the event:
 * $C = \displaystyle \bigcap_{k \mathop = 5}^{10} A_k$

Then $C$ is the event of hitting $T$ inside circle $C_5$.

Proof
By the geometry of the situation:
 * $C_5 \subseteq C_6 \subseteq \cdots \subseteq C_{10}$

By Events One of Which equals Intersection:

The result follows.