Two circles are said to be **concentric** if they have the same center. The following figure shows two concentric circles with the same center O:

Note that any two concentric circles whose radii are *r*_{1} and *r*_{2} (with\({r_1} > {r_2}\)) will form a ring-shaped region between them, as shown below:

Observe that the width of this ring-shaped region will be \({r_1} - {r_2}\).

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