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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
(1/7)*r+r+(1/7)*r = 1/r // - 1/r
(1/7)*r+r+(1/7)*r-(1/r) = 0
(1/7)*r+r+(1/7)*r-r^-1 = 0
9/7*r^1-1*r^-1 = 0
(9/7*r^2-1*r^0)/(r^1) = 0 // * r^2
r^1*(9/7*r^2-1*r^0) = 0
r^1
(9/7)*r^2-1 = 0
(9/7)*r^2-1 = 0
DELTA = 0^2-(-1*4*(9/7))
DELTA = 36/7
DELTA > 0
r = ((36/7)^(1/2)+0)/(2*(9/7)) or r = (0-(36/7)^(1/2))/(2*(9/7))
r = 7/18*(36/7)^(1/2) or r = 7/18*(36/7)^(1/2)
r in { 7/18*(36/7)^(1/2), 7/18*(36/7)^(1/2)}
r in { 7/18*(36/7)^(1/2), 7/18*(36/7)^(1/2) }
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