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D( x )
-3*x = 0
-3*x = 0
-3*x = 0
-3*x = 0 // : -3
x = 0
x in (-oo:0) U (0:+oo)
(9*x^3+3*x^3-(15*x))/(-3*x) = 0
(9*x^3+3*x^3-15*x)/(-3*x) = 0
(12*x^3-15*x)/(-3*x) = 0
12*x^3-15*x = 0
3*x*(4*x^2-5) = 0
4*x^2 = 5 // : 4
x^2 = 5/4
x^2 = 5/4 // ^ 1/2
abs(x) = (5/4)^(1/2)
x = (5/4)^(1/2) or x = -(5/4)^(1/2)
3*x*(x-(5/4)^(1/2))*(x+(5/4)^(1/2)) = 0
(3*x*(x-(5/4)^(1/2))*(x+(5/4)^(1/2)))/(-3*x) = 0
( 3*x )
3*x = 0 // : 3
x = 0
( x+(5/4)^(1/2) )
x+(5/4)^(1/2) = 0 // - (5/4)^(1/2)
x = -(5/4)^(1/2)
( x-(5/4)^(1/2) )
x-(5/4)^(1/2) = 0 // + (5/4)^(1/2)
x = (5/4)^(1/2)
x in { 0}
x in { -(5/4)^(1/2), (5/4)^(1/2) }
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