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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
40 = x+80/x // - x+80/x
40-x-(80/x) = 0
40-x-80*x^-1 = 0
40*x^0-1*x^1-80*x^-1 = 0
(40*x^1-1*x^2-80*x^0)/(x^1) = 0 // * x^2
x^1*(40*x^1-1*x^2-80*x^0) = 0
x^1
40*x-x^2-80 = 0
40*x-x^2-80 = 0
DELTA = 40^2-(-80*(-1)*4)
DELTA = 1280
DELTA > 0
x = (1280^(1/2)-40)/(-1*2) or x = (-1280^(1/2)-40)/(-1*2)
x = (16*5^(1/2)-40)/(-2) or x = (-16*5^(1/2)-40)/(-2)
x in { (16*5^(1/2)-40)/(-2), (-16*5^(1/2)-40)/(-2)}
x in { (16*5^(1/2)-40)/(-2), (-16*5^(1/2)-40)/(-2) }
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