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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
12/x+9 = 13 // - 13
12/x-13+9 = 0
12*x^-1 = 4 // : 12
x^-1 = 1/3
-1 < 0
1/(x^1) = 1/3 // * x^1
1 = 1/3*x^1 // : 1/3
3 = x^1
x = 3
x = 3
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