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D( x )
40*x+40 = 0
6/(40*x+40) = 0
40*x+40 = 0
40*x+40 = 0
40*x+40 = 0 // - 40
40*x = -40 // : 40
x = -40/40
x = -1
6/(40*x+40) = 0
6/(40*x+40) = 0
x in (-oo:-1) U (-1:+oo)
((5*x+5)/3)/(6/(40*x+40)) = 0
((5*x+5)*(40*x+40))/(3*6) = 0
( 40*x+40 )
40*x+40 = 0 // - 40
40*x = -40 // : 40
x = -40/40
x = -1
( 5*x+5 )
5*x+5 = 0 // - 5
5*x = -5 // : 5
x = -5/5
x = -1
x in { -1}
x belongs to the empty set
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