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D( x )
x^2 = 0
x^2 = 0
x^2 = 0
1*x^2 = 0 // : 1
x^2 = 0
x = 0
x in (-oo:0) U (0:+oo)
0 = 12-(192/(x^2)) // - 12-(192/(x^2))
192/(x^2)-12+0 = 0
192/(x^2)-12 = 0
192*x^-2 = 12 // : 192
x^-2 = 1/16
-2 < 0
1/(x^2) = 1/16 // * x^2
1 = 1/16*x^2 // : 1/16
16 = x^2
x^2 = 16 // ^ 1/2
abs(x) = 4
x = 4 or x = -4
x in { 4, -4 }
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