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D( y )
y = 0
y = 0
y = 0
y in (-oo:0) U (0:+oo)
(y^2)/y+y-(90/y)-9-9 = 0
(y^2)/y+y-90*y^-1-9-9 = 0
2*y^1-90*y^-1-18*y^0 = 0
(2*y^2-18*y^1-90*y^0)/(y^1) = 0 // * y^2
y^1*(2*y^2-18*y^1-90*y^0) = 0
y^1
2*y^2-18*y-90 = 0
2*y^2-18*y-90 = 0
DELTA = (-18)^2-(-90*2*4)
DELTA = 1044
DELTA > 0
y = (1044^(1/2)+18)/(2*2) or y = (18-1044^(1/2))/(2*2)
y = (6*29^(1/2)+18)/4 or y = (18-6*29^(1/2))/4
y in { (18-6*29^(1/2))/4, (6*29^(1/2)+18)/4}
y in { (18-6*29^(1/2))/4, (6*29^(1/2)+18)/4 }
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