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D( x )
x*((x-6)/21) = 0
x*((x-6)/21) = 0
x*((x-6)/21) = 0
(x*(x-6))/21 = 0
( x-6 )
x-6 = 0 // + 6
x = 6
( x )
x = 0
x in (-oo:0) U (0:6) U (6:+oo)
((5*x-30)/33)/(x*((x-6)/21)) = 0
(21*(5*x-30))/(33*x*(x-6)) = 0
5*x-30 = 0 // + 30
5*x = 30 // : 5
x = 30/5
x = 6
x in { 6}
x belongs to the empty set
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