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D( y )
y = 0
y = 0
y = 0
y in (-oo:0) U (0:+oo)
1/y = 30/90 // - 30/90
1/y-(30/90) = 0
1/y-1/3 = 0
1*y^-1 = 1/3 // : 1
y^-1 = 1/3
-1 < 0
1/(y^1) = 1/3 // * y^1
1 = 1/3*y^1 // : 1/3
3 = y^1
y = 3
y = 3
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