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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
14/21 = 6/x // - 6/x
14/21-(6/x) = 0
14/21-6*x^-1 = 0
-6*x^-1 = -2/3 // : -6
x^-1 = 1/9
-1 < 0
1/(x^1) = 1/9 // * x^1
1 = 1/9*x^1 // : 1/9
9 = x^1
x = 9
x = 9
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