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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
39.9/x = 2.1 // - 2.1
39.9/x-2.1 = 0
39.9*x^-1 = 2.1 // : 39.9
x^-1 = 0.05263158
-1 < 0
1/(x^1) = 0.05263158 // * x^1
1 = 0.05263158*x^1 // : 0.05263158
18.99999962 = x^1
x = 18.99999962
x = 18.99999962
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