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D( y )
4-y = 0
4-y = 0
4-y = 0
4-y = 0 // - 4
-y = -4 // * -1
y = 4
y in (-oo:4) U (4:+oo)
(y-4)/(4-y) = 0
y-4 = 0 // + 4
y = 4
y in { 4}
y belongs to the empty set
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