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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)
5-(20/(x^2)) = 0
5-20*x^-2 = 0
-20*x^-2 = -5 // : -20
x^-2 = 1/4
-2 < 0
1/(x^2) = 1/4 // * x^2
1 = 1/4*x^2 // : 1/4
4 = x^2
x^2 = 4 // ^ 1/2
abs(x) = 2
x = 2 or x = -2
x in { 2, -2 }
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