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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
4/x = (8/20)*x // - (8/20)*x
4/x-((8/20)*x) = 0
(-2/5)*x+4/x = 0
4*x^-1-2/5*x^1 = 0
(4*x^0-2/5*x^2)/(x^1) = 0 // * x^2
x^1*(4*x^0-2/5*x^2) = 0
x^1
(-2/5)*x^2+4 = 0
(-2/5)*x^2+4 = 0
DELTA = 0^2-(4*4*(-2/5))
DELTA = 32/5
DELTA > 0
x = ((32/5)^(1/2)+0)/(2*(-2/5)) or x = (0-(32/5)^(1/2))/(2*(-2/5))
x = -5/4*(32/5)^(1/2) or x = 5/4*(32/5)^(1/2)
x in { -5/4*(32/5)^(1/2), 5/4*(32/5)^(1/2)}
x in { -5/4*(32/5)^(1/2), 5/4*(32/5)^(1/2) }
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