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D( x )
5*x^2 = 0
5*x^2 = 0
5*x^2 = 0
5*x^2 = 0 // : 5
x^2 = 0
x = 0
x in (-oo:0) U (0:+oo)
(2*x^2-5)/(5*x^2) = 0
2*x^2 = 5 // : 2
x^2 = 5/2
x^2 = 5/2 // ^ 1/2
abs(x) = (5/2)^(1/2)
x = (5/2)^(1/2) or x = -(5/2)^(1/2)
x in { (5/2)^(1/2), -(5/2)^(1/2) }
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