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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
(4*x)/5 = 99/x // - 99/x
(4*x)/5-(99/x) = 0
(4*x)/5-99*x^-1 = 0
4/5*x^1-99*x^-1 = 0
(4/5*x^2-99*x^0)/(x^1) = 0 // * x^2
x^1*(4/5*x^2-99*x^0) = 0
x^1
(4/5)*x^2-99 = 0
(4/5)*x^2-99 = 0
DELTA = 0^2-(-99*4*(4/5))
DELTA = 1584/5
DELTA > 0
x = ((1584/5)^(1/2)+0)/(2*(4/5)) or x = (0-(1584/5)^(1/2))/(2*(4/5))
x = 5/8*(1584/5)^(1/2) or x = 5/8*(1584/5)^(1/2)
x in { 5/8*(1584/5)^(1/2), 5/8*(1584/5)^(1/2)}
x in { 5/8*(1584/5)^(1/2), 5/8*(1584/5)^(1/2) }
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