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x in (-oo:+oo)
x^2*(400-(x*((2*x^2)/3))) = 0
x^2*((-2/3)*x^3+400) = 0
x^2*(400-2/3*x^3) = 0
( 400-2/3*x^3 )
-2/3*x^3 = -400 // : -2/3
x^3 = 600
x^3 = 600 // ^ 1/3
x = 2*75^(1/3)
( x^2 )
1*x^2 = 0 // : 1
x^2 = 0
x = 0
x in { 2*75^(1/3), 0 }
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