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