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D( x )
(8/10)*x^2 = 0
(8/10)*x^2 = 0
(8/10)*x^2 = 0
4/5*x^2 = 0 // : 4/5
x^2 = 0
x = 0
x in (-oo:0) U (0:+oo)
((5*x^2)/4)/((8/10)*x^2) = 0
25/16 = 0
x belongs to the empty set
x belongs to the empty set
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