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D( z )
z = 0
z = 0
z = 0
z in (-oo:0) U (0:+oo)
3/7 = 12/z // - 12/z
3/7-(12/z) = 0
3/7-12*z^-1 = 0
-12*z^-1 = -3/7 // : -12
z^-1 = 1/28
-1 < 0
1/(z^1) = 1/28 // * z^1
1 = 1/28*z^1 // : 1/28
28 = z^1
z = 28
z = 28
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