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D( x )
20*x^3 = 0
20*x^3 = 0
20*x^3 = 0
20*x^3 = 0 // : 20
x^3 = 0
x = 0
x in (-oo:0) U (0:+oo)
(16*x)/(20*x^3) = 0
4/5*x^-2 = 0 // : 4/5
x^-2 = 0
x należy do O
x belongs to the empty set
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