Answer:
circle is not a function.
Explanation:
there are certain rules to define a mapping as function, one of them is function should not have more than 1 value at a given x.
for illustration purpose consider the below diagram,
as you can see in the given circle at x = 2, there are two different values of y, therefore equation of circle doesn't qualify as a function. half circle can be defined as a function (by restricting the range to only positive or negative values).
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