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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
4*r-(4/r) = 8/5 // - 8/5
4*r-(4/r)-(8/5) = 0
4*r-4*r^-1-8/5 = 0
4*r^1-4*r^-1-8/5*r^0 = 0
(4*r^2-8/5*r^1-4*r^0)/(r^1) = 0 // * r^2
r^1*(4*r^2-8/5*r^1-4*r^0) = 0
r^1
4*r^2+(-8/5)*r-4 = 0
4*r^2+(-8/5)*r-4 = 0
DELTA = (-8/5)^2-(-4*4*4)
DELTA = 1664/25
DELTA > 0
r = ((1664/25)^(1/2)-(-8/5))/(2*4) or r = (-(-8/5)-(1664/25)^(1/2))/(2*4)
r = ((1664/25)^(1/2)+8/5)/8 or r = (8/5-(1664/25)^(1/2))/8
r in { (8/5-(1664/25)^(1/2))/8, ((1664/25)^(1/2)+8/5)/8}
r in { (8/5-(1664/25)^(1/2))/8, ((1664/25)^(1/2)+8/5)/8 }
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