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D( f )
f = 0
f = 0
f = 0
f in (-oo:0) U (0:+oo)
72/48 = 6/f // - 6/f
72/48-(6/f) = 0
72/48-6*f^-1 = 0
-6*f^-1 = -3/2 // : -6
f^-1 = 1/4
-1 < 0
1/(f^1) = 1/4 // * f^1
1 = 1/4*f^1 // : 1/4
4 = f^1
f = 4
f = 4
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