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D( x )
((4*x)/5)*x^3 = 0
((4*x)/5)*x^3 = 0
((4*x)/5)*x^3 = 0
4/5*x^4 = 0 // : 4/5
x^4 = 0
x = 0
x in (-oo:0) U (0:+oo)
(x^2*((2*x^7)/5))/(((4*x)/5)*x^3) = 0
1/2*x^5 = 0 // : 1/2
x^5 = 0
x = 0
x in { 0}
x belongs to the empty set
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