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D( f )
f = 0
f = 0
f = 0
f in (-oo:0) U (0:+oo)
90/75 = 6/f // - 6/f
90/75-(6/f) = 0
90/75-6*f^-1 = 0
-6*f^-1 = -6/5 // : -6
f^-1 = 1/5
-1 < 0
1/(f^1) = 1/5 // * f^1
1 = 1/5*f^1 // : 1/5
5 = f^1
f = 5
f = 5
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