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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
12/x = 3.5 // - 3.5
12/x-3.5 = 0
12*x^-1 = 3.5 // : 12
x^-1 = 0.29166667
-1 < 0
1/(x^1) = 0.29166667 // * x^1
1 = 0.29166667*x^1 // : 0.29166667
3.42857139 = x^1
x = 3.42857139
x = 3.42857139
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