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