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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)
(x^5-x)/(x^4) = 0
x^5-x = 0
x*(x^4-1) = 0
1*x^4 = 1 // : 1
x^4 = 1
x^4 = 1 // ^ 1/4
abs(x) = 1
x = 1 or x = -1
x*(x-1)*(x+1) = 0
(x*(x-1)*(x+1))/(x^4) = 0
( x+1 )
x+1 = 0 // - 1
x = -1
( x-1 )
x-1 = 0 // + 1
x = 1
( x )
x = 0
x in { 0}
x in { -1, 1 }
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