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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
28/x = 7/3 // - 7/3
28/x-(7/3) = 0
28/x-7/3 = 0
28*x^-1 = 7/3 // : 28
x^-1 = 1/12
-1 < 0
1/(x^1) = 1/12 // * x^1
1 = 1/12*x^1 // : 1/12
12 = x^1
x = 12
x = 12
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