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x in (-oo:+oo)
300-((x^2)/30) = 0
-1/30*x^2 = -300 // : -1/30
x^2 = 9000
x^2 = 9000 // ^ 1/2
abs(x) = 30*10^(1/2)
x = 30*10^(1/2) or x = -(30*10^(1/2))
x in { 30*10^(1/2), -(30*10^(1/2)) }
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