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D( x )
-x-3 = 0
-x-3 = 0
-x-3 = 0
-x-3 = 0 // + 3
-x = 3 // * -1
x = -3
x in (-oo:-3) U (-3:+oo)
(((x-12)*(x+3)*(x+6)*(2*x+3))/(-x-3))*(6-x)*(-2*x-3)*(12-x) = 0
((x-12)*(x+3)*(x+6)*(2*x+3)*(6-x)*(-2*x-3)*(12-x))/(-x-3) = 0
( -2*x-3 )
-2*x-3 = 0 // + 3
-2*x = 3 // : -2
x = 3/(-2)
x = -3/2
( 12-x )
12-x = 0 // - 12
-x = -12 // * -1
x = 12
( 2*x+3 )
2*x+3 = 0 // - 3
2*x = -3 // : 2
x = -3/2
( 6-x )
6-x = 0 // - 6
-x = -6 // * -1
x = 6
( x+3 )
x+3 = 0 // - 3
x = -3
( x+6 )
x+6 = 0 // - 6
x = -6
( x-12 )
x-12 = 0 // + 12
x = 12
x in { -3}
x in { -3/2, 12, -3/2, 6, -6, 12 }
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