Question

is square root x^4-4-y=0

and x^2+y^2=4

a function

Answer 1

square root x^4-4-y=0

y = √(x^4-4) , yes , a function

x^2 + y^2 = 4 is not a function, it is the equation of a circle, so clearly for an x you pick in the domain there is more than one value of y