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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
9/4 = r-(10/r) // - r-(10/r)
10/r-r+9/4 = 0
10*r^-1-1*r^1+9/4*r^0 = 0
(9/4*r^1-1*r^2+10*r^0)/(r^1) = 0 // * r^2
r^1*(9/4*r^1-1*r^2+10*r^0) = 0
r^1
(9/4)*r-r^2+10 = 0
(9/4)*r-r^2+10 = 0
DELTA = (9/4)^2-(-1*4*10)
DELTA = 721/16
DELTA > 0
r = ((721/16)^(1/2)-(9/4))/(-1*2) or r = (-(9/4)-(721/16)^(1/2))/(-1*2)
r = ((721/16)^(1/2)-9/4)/(-2) or r = ((721/16)^(1/2)+9/4)/2
r in { ((721/16)^(1/2)-9/4)/(-2), ((721/16)^(1/2)+9/4)/2}
r in { ((721/16)^(1/2)-9/4)/(-2), ((721/16)^(1/2)+9/4)/2 }
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