#### Answer

small circle: $x^2+y^2=9$
large circle: $x^2+y^2=25$

#### Work Step by Step

Step 1. Using the figure in the exercise and given conditions, we have $a=5, b=3$, centered at $(0,0)$ for all curves.
Step 2. For the small circle (green), we have $r=b=3$; thus the equation is $x^2+y^2=9$
Step 3. For the large circle (blue), we have $r=a=5$; thus the equation is $x^2+y^2=25$