the circles x^2 +y^2 =5 and x^2 +y^2 =10? If so, what is an example? If not, why not?
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z = (x^2 + y^2 - 5) (x^2 + y^2 - 10)
at z=0
(x^2 + y^2 - 5) (x^2 + y^2 - 10) = 0
x^2 + y^2 - 5 = 0
x^2 + y^2 - 10 = 0
at z=0
(x^2 + y^2 - 5) (x^2 + y^2 - 10) = 0
x^2 + y^2 - 5 = 0
x^2 + y^2 - 10 = 0