Describe carefully the largest subset(s) of a circle that are functions.

[boucadam243]

Question:
Describe carefully the largest subset(s) of a circle that are functions.

Really need the help. I understand subsets,but I don't understand the question.
Thank you

 

[soroban]

Describe carefully the largest subset(s) of a circle that are functions.

The author is apparently being charged $10 per word.
He/she could have been much clearer.
Here is my interpretation of the problem.

A circle is not a function.
\(\displaystyle x^2 + y^2 \,=\,r^2 \quad\Rightarrow\quad y \,=\,\pm\sqrt{r^2-x^2}\)

But, say, the upper semicircle is a function: \(\displaystyle y \,=\,\sqrt{r^2-x^2}\)