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x in (-oo:+oo)
(x/3)*x = 15 // - 15
(x/3)*x-15 = 0
1/3*x^2 = 15 // : 1/3
x^2 = 45
x^2 = 45 // ^ 1/2
abs(x) = 3*5^(1/2)
x = 3*5^(1/2) or x = -(3*5^(1/2))
x in { 3*5^(1/2), -(3*5^(1/2)) }
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