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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
40*x-39 < 40/x // - 40/x
40*x-(40/x)-39 < 0
40*x-40*x^-1-39 < 0
40*x-40*x^-1-39 = 0
40*x^1-40*x^-1-39*x^0 = 0
(40*x^2-39*x^1-40*x^0)/(x^1) = 0 // * x^2
x^1*(40*x^2-39*x^1-40*x^0) = 0
x^1
40*x^2-39*x-40 = 0
40*x^2-39*x-40 = 0
DELTA = (-39)^2-(-40*4*40)
DELTA = 7921
DELTA > 0
x = (7921^(1/2)+39)/(2*40) or x = (39-7921^(1/2))/(2*40)
x = 8/5 or x = -5/8
x in { -5/8, 8/5}
(x+5/8)*(x-8/5) < 0
(-oo:-5/8) (-5/8:8/5) (8/5:+oo) x+5/8 - + + x-8/5 - - +
x in (-5/8:0) U (0:8/5)
(-5/8:0) U (0:8/5)
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