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D( x )
(7*x^3)/13 = 0
(7*x^3)/13 = 0
(7*x^3)/13 = 0
7/13*x^3 = 0 // : 7/13
x^3 = 0
x = 0
x in (-oo:0) U (0:+oo)
(49*x^6)/((7*x^3)/13) = 0
91*x^3 = 0 // : 91
x^3 = 0
x = 0
x in { 0}
x belongs to the empty set
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