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