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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
12/10 = 30/x // - 30/x
12/10-(30/x) = 0
12/10-30*x^-1 = 0
-30*x^-1 = -6/5 // : -30
x^-1 = 1/25
-1 < 0
1/(x^1) = 1/25 // * x^1
1 = 1/25*x^1 // : 1/25
25 = x^1
x = 25
x = 25
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