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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
(4*x)/25 = 4/x // - 4/x
(4*x)/25-(4/x) = 0
(4*x)/25-4*x^-1 = 0
4/25*x^1-4*x^-1 = 0
(4/25*x^2-4*x^0)/(x^1) = 0 // * x^2
x^1*(4/25*x^2-4*x^0) = 0
x^1
(4/25)*x^2-4 = 0
(4/25)*x^2-4 = 0
DELTA = 0^2-(-4*4*(4/25))
DELTA = 64/25
DELTA > 0
x = ((64/25)^(1/2)+0)/(2*(4/25)) or x = (0-(64/25)^(1/2))/(2*(4/25))
x = 5 or x = -5
x in { -5, 5}
x in { -5, 5 }
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