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D( f )
f = 0
f = 0
f = 0
f in (-oo:0) U (0:+oo)
2.5 = f/f-15 // - f/f-15
2.5-(f/f)+15 = 0
2.5-1+15 = 0
16.5 = 0
f belongs to the empty set
f belongs to the empty set
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