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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
30/66 = z/x // - z/x
30/66-(z/x) = 0
x^-1*(-z) = -5/11 // : -z
x^-1 = 5/11/z
-1 < 0
1/(x^1) = 5/11/z // * x^1
1 = x^1*(5/11/z) // : 5/11/z
11/5*z = x^1
x = 11/5*z
x = 11/5*z
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