## Calculus, 10th Edition (Anton)

Yes. $$y=\sqrt{25-x^2}$$
Yes, it is a function since there exists no real number for which there are two distinct values of $y$. This graph is the positive section of the circle$$x^2+y^2=25 \quad \Rightarrow \quad y= \pm \sqrt{25-x^2},$$so the function representing this section is $$y=\sqrt{25-x^2}.$$