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D( x )
x^4+1 = 0
x^4+1 = 0
x^4+1 = 0
1*x^4 = -1 // : 1
x^4 = -1
x in (-oo:+oo)
(-2*x)/(x^4+1) = 0
-2*x = 0 // : -2
x = 0
x = 0
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