Question 699349
Determine whether the equation defines y as a function of x. 
x+y=9
y = -x+9
y is a function of x.
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x^2+y^2=1
y = +-sqrt(-x^2+1)
y is not a function as there are two y-value for some values of "x".

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and 
x=y^2
y = +-sqrt(x)
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Cheers,
Stan H.