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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
((25*r^7*x^6)/20)*r^3*x^-4 = 0
5/4*r^10*x^2 = 0 // : 5/4*r^10
x^2 = 0
x = 0
x in { 0}
x belongs to the empty set
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