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D( y )
y = 0
y = 0
y = 0
y in (-oo:0) U (0:+oo)
9*y+7/y-12 = 0
9*y^1+7*y^-1-12*y^0 = 0
(9*y^2-12*y^1+7*y^0)/(y^1) = 0 // * y^2
y^1*(9*y^2-12*y^1+7*y^0) = 0
y^1
9*y^2-12*y+7 = 0
9*y^2-12*y+7 = 0
DELTA = (-12)^2-(4*7*9)
DELTA = -108
DELTA < 0
y in { }
y belongs to the empty set
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