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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
1/6 = r-(19/r) // - r-(19/r)
19/r-r+1/6 = 0
19*r^-1-1*r^1+1/6*r^0 = 0
(1/6*r^1-1*r^2+19*r^0)/(r^1) = 0 // * r^2
r^1*(1/6*r^1-1*r^2+19*r^0) = 0
r^1
(1/6)*r-r^2+19 = 0
(1/6)*r-r^2+19 = 0
DELTA = (1/6)^2-(-1*4*19)
DELTA = 2737/36
DELTA > 0
r = ((2737/36)^(1/2)-(1/6))/(-1*2) or r = (-(1/6)-(2737/36)^(1/2))/(-1*2)
r = ((2737/36)^(1/2)-1/6)/(-2) or r = ((2737/36)^(1/2)+1/6)/2
r in { ((2737/36)^(1/2)-1/6)/(-2), ((2737/36)^(1/2)+1/6)/2}
r in { ((2737/36)^(1/2)-1/6)/(-2), ((2737/36)^(1/2)+1/6)/2 }
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