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