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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
1.4*(30/x) = 100 // - 100
1.4*(30/x)-100 = 0
42*x^-1 = 100 // : 42
x^-1 = 50/21
-1 < 0
1/(x^1) = 50/21 // * x^1
1 = 50/21*x^1 // : 50/21
21/50 = x^1
x = 21/50
x = 21/50
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