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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
5*x-(20/x)-4 = 9 // - 9
5*x-(20/x)-9-4 = 0
5*x-20*x^-1-9-4 = 0
5*x^1-20*x^-1-13*x^0 = 0
(5*x^2-13*x^1-20*x^0)/(x^1) = 0 // * x^2
x^1*(5*x^2-13*x^1-20*x^0) = 0
x^1
5*x^2-13*x-20 = 0
5*x^2-13*x-20 = 0
DELTA = (-13)^2-(-20*4*5)
DELTA = 569
DELTA > 0
x = (569^(1/2)+13)/(2*5) or x = (13-569^(1/2))/(2*5)
x = (569^(1/2)+13)/10 or x = (13-569^(1/2))/10
x in { (13-569^(1/2))/10, (569^(1/2)+13)/10}
x in { (13-569^(1/2))/10, (569^(1/2)+13)/10 }
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