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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
D = m/x // - m/x
D-(m/x) = 0
-m*x^-1 = -D // : -m
x^-1 = D/m
-1 < 0
1/(x^1) = D/m // * x^1
1 = (D/m)*x^1 // : D/m
1/(D/m) = x^1
x = 1/(D/m)
x = 1/(D/m)
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