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x in (-oo:+oo)
x*((12*x^3-(21*x))/3) = 0
x*((12*x^3-21*x)/3) = 0
(x*(12*x^3-21*x))/3 = 0
12*x^3-21*x = 0
3*x*(4*x^2-7) = 0
4*x^2 = 7 // : 4
x^2 = 7/4
x^2 = 7/4 // ^ 1/2
abs(x) = (7/4)^(1/2)
x = (7/4)^(1/2) or x = -(7/4)^(1/2)
3*x*(x-(7/4)^(1/2))*(x+(7/4)^(1/2)) = 0
(3*x*x*(x-(7/4)^(1/2))*(x+(7/4)^(1/2)))/3 = 0
( 3*x )
3*x = 0 // : 3
x = 0
( x+(7/4)^(1/2) )
x+(7/4)^(1/2) = 0 // - (7/4)^(1/2)
x = -(7/4)^(1/2)
( x-(7/4)^(1/2) )
x-(7/4)^(1/2) = 0 // + (7/4)^(1/2)
x = (7/4)^(1/2)
( x )
x = 0
x in { 0, -(7/4)^(1/2), (7/4)^(1/2), 0 }
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