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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
(9*x)/x-(2*x)-(21/x)+3+3 = 0
(9*x)/x-2*x-21*x^-1+3+3 = 0
15*x^0-2*x^1-21*x^-1 = 0
(15*x^1-2*x^2-21*x^0)/(x^1) = 0 // * x^2
x^1*(15*x^1-2*x^2-21*x^0) = 0
x^1
15*x-2*x^2-21 = 0
15*x-2*x^2-21 = 0
DELTA = 15^2-(-21*(-2)*4)
DELTA = 57
DELTA > 0
x = (57^(1/2)-15)/(-2*2) or x = (-57^(1/2)-15)/(-2*2)
x = (57^(1/2)-15)/(-4) or x = (57^(1/2)+15)/4
x in { (57^(1/2)-15)/(-4), (57^(1/2)+15)/4}
x in { (57^(1/2)-15)/(-4), (57^(1/2)+15)/4 }
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