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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
12/r-((1/21)*r) = r-(8/3) // - r-(8/3)
12/r-((1/21)*r)-r+8/3 = 0
(-1/21)*r-r+12/r+8/3 = 0
12*r^-1-22/21*r^1+8/3*r^0 = 0
(8/3*r^1-22/21*r^2+12*r^0)/(r^1) = 0 // * r^2
r^1*(8/3*r^1-22/21*r^2+12*r^0) = 0
r^1
(-22/21)*r^2+(8/3)*r+12 = 0
(-22/21)*r^2+(8/3)*r+12 = 0
DELTA = (8/3)^2-(4*12*(-22/21))
DELTA = 3616/63
DELTA > 0
r = ((3616/63)^(1/2)-(8/3))/(2*(-22/21)) or r = (-(8/3)-(3616/63)^(1/2))/(2*(-22/21))
r = -21/44*((3616/63)^(1/2)-8/3) or r = 21/44*((3616/63)^(1/2)+8/3)
r in { -21/44*((3616/63)^(1/2)-8/3), 21/44*((3616/63)^(1/2)+8/3)}
r in { -21/44*((3616/63)^(1/2)-8/3), 21/44*((3616/63)^(1/2)+8/3) }
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