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D( x )
(39*x)/(-40) = 0
(39*x)/(-40) = 0
(39*x)/(-40) = 0
-39/40*x = 0 // : -39/40
x = 0
x in (-oo:0) U (0:+oo)
((-26*x^2)/35)/((39*x)/(-40)) = 0
16/21*x = 0 // : 16/21
x = 0
x in { 0}
x belongs to the empty set
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