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D( x )
x = 0
x^2 = 0
x = 0
x = 0
x^2 = 0
x^2 = 0
1*x^2 = 0 // : 1
x^2 = 0
x = 0
x in (-oo:0) U (0:+oo)
4/x-(5/(x^2)) = 15 // - 15
4/x-(5/(x^2))-15 = 0
4/x-5*x^-2-15 = 0
4*x^-1-5*x^-2-15 = 0
t_1 = x^-1
4*t_1^1-5*t_1^2-15 = 0
4*t_1-5*t_1^2-15 = 0
DELTA = 4^2-(-15*(-5)*4)
DELTA = -284
DELTA < 0
x belongs to the empty set
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