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D( f )
f = 0
f = 0
f = 0
f in (-oo:0) U (0:+oo)
5/15 = 7/f // - 7/f
5/15-(7/f) = 0
5/15-7*f^-1 = 0
-7*f^-1 = -1/3 // : -7
f^-1 = 1/21
-1 < 0
1/(f^1) = 1/21 // * f^1
1 = 1/21*f^1 // : 1/21
21 = f^1
f = 21
f = 21
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