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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
3/8 = 21/x // - 21/x
3/8-(21/x) = 0
3/8-21*x^-1 = 0
-21*x^-1 = -3/8 // : -21
x^-1 = 1/56
-1 < 0
1/(x^1) = 1/56 // * x^1
1 = 1/56*x^1 // : 1/56
56 = x^1
x = 56
x = 56
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