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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
6*x+5 = 21/x // - 21/x
6*x-(21/x)+5 = 0
6*x-21*x^-1+5 = 0
6*x^1-21*x^-1+5*x^0 = 0
(6*x^2+5*x^1-21*x^0)/(x^1) = 0 // * x^2
x^1*(6*x^2+5*x^1-21*x^0) = 0
x^1
6*x^2+5*x-21 = 0
6*x^2+5*x-21 = 0
DELTA = 5^2-(-21*4*6)
DELTA = 529
DELTA > 0
x = (529^(1/2)-5)/(2*6) or x = (-529^(1/2)-5)/(2*6)
x = 3/2 or x = -7/3
x in { -7/3, 3/2}
x in { -7/3, 3/2 }
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