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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
8/12 = 10/x // - 10/x
8/12-(10/x) = 0
8/12-10*x^-1 = 0
-10*x^-1 = -2/3 // : -10
x^-1 = 1/15
-1 < 0
1/(x^1) = 1/15 // * x^1
1 = 1/15*x^1 // : 1/15
15 = x^1
x = 15
x = 15
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