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D( x )
x^2 = 0
x^2 = 0
x^2 = 0
1*x^2 = 0 // : 1
x^2 = 0
x = 0
x in (-oo:0) U (0:+oo)
0.9-(200/(x^2)) = 0
0.9-200*x^-2 = 0
-200*x^-2 = -0.9 // : -200
x^-2 = 0.0045
-2 < 0
1/(x^2) = 0.0045 // * x^2
1 = 0.0045*x^2 // : 0.0045
222.22222222 = x^2
x^2 = 222.22222222 // ^ 1/2
abs(x) = 222.22222222^(1/2)
x = 222.22222222^(1/2) or x = -222.22222222^(1/2)
x in { 222.22222222^(1/2), -222.22222222^(1/2) }
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