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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
4/12 = 7/x // - 7/x
4/12-(7/x) = 0
4/12-7*x^-1 = 0
-7*x^-1 = -1/3 // : -7
x^-1 = 1/21
-1 < 0
1/(x^1) = 1/21 // * x^1
1 = 1/21*x^1 // : 1/21
21 = x^1
x = 21
x = 21
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