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