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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)
2 = 400/(x^2) // - 400/(x^2)
2-(400/(x^2)) = 0
2-400*x^-2 = 0
-400*x^-2 = -2 // : -400
x^-2 = 1/200
-2 < 0
1/(x^2) = 1/200 // * x^2
1 = 1/200*x^2 // : 1/200
200 = x^2
x^2 = 200 // ^ 1/2
abs(x) = 10*2^(1/2)
x = 10*2^(1/2) or x = -(10*2^(1/2))
x in { 10*2^(1/2), -(10*2^(1/2)) }
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