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D( x )
(10*x)/21 = 0
(10*x)/21 = 0
(10*x)/21 = 0
10/21*x = 0 // : 10/21
x = 0
x in (-oo:0) U (0:+oo)
((5*x^2)/3)/((10*x)/21) = 0
7/2*x = 0 // : 7/2
x = 0
x in { 0}
x belongs to the empty set
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