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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
12 = 3/x // - 3/x
12-(3/x) = 0
12-3*x^-1 = 0
-3*x^-1 = -12 // : -3
x^-1 = 4
-1 < 0
1/(x^1) = 4 // * x^1
1 = 4*x^1 // : 4
1/4 = x^1
x = 1/4
x = 1/4
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