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D( f )
f = 0
f = 0
f = 0
f in (-oo:0) U (0:+oo)
5/f-(35/21) = 0
5/f-5/3 = 0
5*f^-1 = 5/3 // : 5
f^-1 = 1/3
-1 < 0
1/(f^1) = 1/3 // * f^1
1 = 1/3*f^1 // : 1/3
3 = f^1
f = 3
f = 3
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