# 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:

$C = \ds \bigcap_{k \mathop = 5}^{10} A_k$

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

## Proof

By the geometry of the situation:

$C_5 \subseteq C_6 \subseteq \cdots \subseteq C_{10}$
 $\ds A_5 \cap A_{10} = A_5$ $\iff$ $\ds A_5 \subseteq A_{10}$ $\ds A_5 \cap A_9 = A_5$ $\iff$ $\ds A_5 \subseteq A_9$ $\ds$ $\vdots$ $\ds$ $\ds A_5 \cap A_6 = A_5$ $\iff$ $\ds A_5 \subseteq A_6$

The result follows.

$\blacksquare$

## Sources

Note that the question does not state whether $r_k < r_{k + 1}$ for all $k = 1, 2, \ldots, 9$ or $r_k > r_{k + 1}$ for all $k = 1, 2, \ldots, 9$. The interpretation made here is the one which provides the correct answer.