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D( x )
x = 0
x+9 = 0
x = 0
x = 0
x+9 = 0
x+9 = 0
x+9 = 0 // - 9
x = -9
x in (-oo:-9) U (-9:0) U (0:+oo)
10/(x+9)-(10/x) = 2.5 // - 2.5
10/(x+9)-(10/x)-2.5 = 0
10/(x+9)-10*x^-1-2.5 = 0
10/(x+9)-10/x-2.5 = 0
(10*x)/(x*(x+9))+(-10*(x+9))/(x*(x+9))+(-2.5*x*(x+9))/(x*(x+9)) = 0
10*x-10*(x+9)-2.5*x*(x+9) = 0
-2.5*x^2-22.5*x-90 = 0
-2.5*x^2-22.5*x-90 = 0
-1*(2.5*x^2+22.5*x+90) = 0
2.5*x^2+22.5*x+90 = 0
DELTA = 22.5^2-(2.5*4*90)
DELTA = 22.5^2-900
DELTA = -393.75
DELTA < 0
-1 = 0
-1/(x*(x+9)) = 0
-1/(x*(x+9)) = 0 // * x*(x+9)
-1 = 0
x belongs to the empty set
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