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D( f )
f = 0
f = 0
f = 0
f in (-oo:0) U (0:+oo)
16/f = -7 // + 7
16/f+7 = 0
16*f^-1 = -7 // : 16
f^-1 = -7/16
-1 < 0
1/(f^1) = -7/16 // * f^1
1 = -7/16*f^1 // : -7/16
-16/7 = f^1
f = -16/7
f = -16/7
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