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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)
(51*x^3-(42*x))/(-3*x) = 0
(51*x^3-42*x)/(-3*x) = 0
51*x^3-42*x = 0
3*x*(17*x^2-14) = 0
17*x^2 = 14 // : 17
x^2 = 14/17
x^2 = 14/17 // ^ 1/2
abs(x) = (14/17)^(1/2)
x = (14/17)^(1/2) or x = -(14/17)^(1/2)
3*x*(x-(14/17)^(1/2))*(x+(14/17)^(1/2)) = 0
(3*x*(x-(14/17)^(1/2))*(x+(14/17)^(1/2)))/(-3*x) = 0
( 3*x )
3*x = 0 // : 3
x = 0
( x+(14/17)^(1/2) )
x+(14/17)^(1/2) = 0 // - (14/17)^(1/2)
x = -(14/17)^(1/2)
( x-(14/17)^(1/2) )
x-(14/17)^(1/2) = 0 // + (14/17)^(1/2)
x = (14/17)^(1/2)
x in { 0}
x in { -(14/17)^(1/2), (14/17)^(1/2) }
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