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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
42/x = 6/7 // - 6/7
42/x-(6/7) = 0
42/x-6/7 = 0
42*x^-1 = 6/7 // : 42
x^-1 = 1/49
-1 < 0
1/(x^1) = 1/49 // * x^1
1 = 1/49*x^1 // : 1/49
49 = x^1
x = 49
x = 49
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