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D( x )
x^2+1 = 0
x^2+1 = 0
x^2+1 = 0
1*x^2 = -1 // : 1
x^2 = -1
x in (-oo:+oo)
(18-6*x)/(x^2+1) = 6 // - 6
(18-6*x)/(x^2+1)-6 = 0
(18-6*x)/(x^2+1)+(-6*(x^2+1))/(x^2+1) = 0
18-6*(x^2+1)-6*x = 0
12-6*x^2-6*x = 0
12-6*x^2-6*x = 0
6*(2-x^2-x) = 0
2-x^2-x = 0
DELTA = (-1)^2-(-1*2*4)
DELTA = 9
DELTA > 0
x = (9^(1/2)+1)/(-1*2) or x = (1-9^(1/2))/(-1*2)
x = -2 or x = 1
6*(x+2)*(x-1) = 0
(6*(x+2)*(x-1))/(x^2+1) = 0
(6*(x+2)*(x-1))/(x^2+1) = 0 // * x^2+1
6*(x+2)*(x-1) = 0
( x+2 )
x+2 = 0 // - 2
x = -2
( x-1 )
x-1 = 0 // + 1
x = 1
x in { -2, 1 }
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