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D( x )
x+3 = 0
(x-3)*(x+3) = 0
x+3 = 0
x+3 = 0
x+3 = 0 // - 3
x = -3
(x-3)*(x+3) = 0
(x-3)*(x+3) = 0
( x+3 )
x+3 = 0 // - 3
x = -3
( x-3 )
x-3 = 0 // + 3
x = 3
x in (-oo:-3) U (-3:3) U (3:+oo)
(5*x)/((x-3)*(x+3)) = 15/((x-3)*(x+3))-(4/(x+3)) // - 15/((x-3)*(x+3))-(4/(x+3))
(5*x)/((x-3)*(x+3))-(15/((x-3)*(x+3)))+4/(x+3) = 0
(5*x)/((x-3)*(x+3))-15*(x-3)^-1*(x+3)^-1+4/(x+3) = 0
(5*x)/((x-3)*(x+3))-15/((x-3)*(x+3))+4/(x+3) = 0
(5*x)/((x-3)*(x+3))-15/((x-3)*(x+3))+(4*(x-3))/((x-3)*(x+3)) = 0
4*(x-3)+5*x-15 = 0
5*x+4*x-15-12 = 0
9*x-27 = 0
(9*x-27)/((x-3)*(x+3)) = 0
(9*x-27)/((x-3)*(x+3)) = 0 // * (x-3)*(x+3)
9*x-27 = 0
9*x-27 = 0 // + 27
9*x = 27 // : 9
x = 27/9
x = 3
x in { 3}
x belongs to the empty set
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